[wrxtasy]

OK, by setting 'Adjust display refresh rate' to ON I workarounded (not solved) the issue. 

You can test by yourself:
1. Disable any "Motion Interpolation" on the TV
2. Set Kodi to 3840 × 2160 pixel @ 60 Hz
3. Set 'Adjust display refresh rate' to OFF
4. Play a 23.976 fps video

I hope that the NOT smooth playback will be solved also if the setting 'Adjust display refresh rate' is setted to OFF.

Thanks!

Kodi is working exactly how it was designed to do with those settings. Adjust Display refresh rate ON is exactly what is needed to match a TV's refresh rate to the fps of source video content. (Provided your media player implements this properly)
You really should be using Adjust Display refresh rate Start/Stop - otherwise as soon as you pullup an On Screen Display while video is playing in background, the TV will re-refresh switch back to the Kodi GUI refresh rate all the time.

Turning it OFF - You are experiencing 3:2 Pull down and common video judder with that setup:
Some self reading for you:

https://www.rtings.com/tv/tests/motion/24p

On Intel machines for example you can set a Kodi GUI of 1080p50Hz and turn "Sync Playback to display On"

23.976fps video will then be sped up to 25fps, audio will be resampled (so no passthrough) and with a bit of TV Motion smoothing - smooth video playback can be the end result.

Still not as good as using - Kodi's - Adjust Display refresh rate Start/Stop and auto switching a TV into it's 24p Cinema or Movie mode.

[outcave]

OK, by setting 'Adjust display refresh rate' to ON I workarounded (not solved) the issue. 

You can test by yourself:
1. Disable any "Motion Interpolation" on the TV
2. Set Kodi to 3840 × 2160 pixel @ 60 Hz
3. Set 'Adjust display refresh rate' to OFF
4. Play a 23.976 fps video

I hope that the NOT smooth playback will be solved also if the setting 'Adjust display refresh rate' is setted to OFF.

Thanks!

Kodi is working exactly how it was designed to do with those settings. Adjust Display refresh rate ON is exactly what is needed to match a TV's refresh rate to the fps of source video content. (Provided your media player implements this properly)
You really should be using Adjust Display refresh rate Start/Stop - otherwise as soon as you pullup an On Screen Display while video is playing in background, the TV will re-refresh switch back to the Kodi GUI refresh rate all the time.

Turning it OFF - You are experiencing 3:2 Pull down and common video judder with that setup:
Some self reading for you:

https://www.rtings.com/tv/tests/motion/24p

On Intel machines for example you can set a Kodi GUI of 1080p50Hz and turn "Sync Playback to display On"

23.976fps video will then be sped up to 25fps, audio will be resampled (so no passthrough) and with a bit of TV Motion smoothing - smooth video playback can be the end result.

Still not as good as using - Kodi's - Adjust Display refresh rate Start/Stop and auto switching a TV into it's 24p Cinema or Movie mode.  

It seems that I'm not the only one to have this behavior (issue for me).
Issue for me because with Kodi 17.6 the video is smooth also playing a 23.976 fps video also if  'Adjust display refresh rate' is setted to OFF (with Kodi always setted to 60 Hz).

More, with LibreELEC 8.2.2 (Kodi 17.6) the video is smooth also playing a 23.976 fps video also if  'Adjust display refresh rate' is setted to OFF (with Kodi always setted to 60 Hz) on Odroid C2 and on Raspberry Pi 2....

I'm sorry but this issue it's related to Kodi 18 and not to the TV.

How is it possible that the same video (23.976 fps) with the same Nvidia Shield with the same HDMI port with same refresh rate (60 Hz) is smooth with PLEX?

How is it possible that the same video (23.976 fps) with the same TV with the same HDMI port with same refresh rate (60 Hz) is smooth with Odroid C2?

I think some is changed in Kodi 18 to VideoPlayer.

[Vispen74]

Kodi 18 Nightly build of January 19th is working great with beta build 1560 of the Mi box.

Edit: Nothing seemed to work well on the Nougat beta 1560-build. Back to stable Marshmallow and trying to get Kodi 18 to run smoothly. I can't make it without Kodi skipping frames again when using Tvheadend PVR add on, regular movies and even recordings seems to play fine.

Any Ideas of settings or a good working build for the Mi Box 3?

[Kalroth]

I just installed the latest nightly build and still have this issue, but way more subtle.

My setup; 

LG OLED 65EF950V
- LG TRU-Motion is completely off
Pioneer SC-LX56 receiver
Nvidia Shield (2015) with latest firmware
Kodi 18 Leia latest nightly build
- 'Adjust display refresh rate' is set to ON (Start/Stop)
- 'Audio Delay' set to -0.150ms for tiptop sync 

In normal spoken scenes you cant really tell the slight stutter/frame skip but when panning or action scenes you can.
In 17.6 it was much more noticable then 18 but its still there, can other confirm?

People with LG OLED tv's having the same issue?

I am experiencing the very same issue on my LG C7 55" OLED, but I am using LibreElec with Kodi 17 on an AMLogic S912 box and with Pioneer VSX-832 AVR.
It is especially noticeable during end credits where the text is very jerky. It is smooth for like 1 second then it stutters/jerks once and is fine again for 1 second, repeat.

EDIT: I found a fix (for me anyways). It seems like outputting at 23.975 fps caused the stuttering on the LG C7.
The stutter is gone ever since I've switched to 60 fps output (disabled adjust refresh rate) from the AMLogic S912 box.
It seems like the LG TV doesn't handle ~24fps very well, at least not with the firmware I was running; 04.70.30 (EU).

I did try booting into Android 7.1.3 and I saw the same kind of judder when outputting ~24fps, which pretty much left the TV as the culprit.
(I'm back to LibreELEC now though, as it's the only thing that'll properly output 4k HDR with Atmos/TrueHD for me)

[wrxtasy]

Kodi 18 Nightly build of January 19th is working great with beta build 1560 of the Mi box.

Edit: Nothing seemed to work well on the Nougat beta 1560-build. Back to stable Marshmallow and trying to get Kodi 18 to run smoothly. I can't make it without Kodi skipping frames again when using Tvheadend PVR add on, regular movies and even recordings seems to play fine.

Any Ideas of settings or a good working build for the Mi Box 3?

Mediacodec hardware decoding on AMLogic boxes running any version of Android Kodi Krypton and using TvHeadend has lousy quality broken TV deinterlacing. When it sorta works it's half motion and stutters.
It's a know AMLogic issue with that type of hardware when using Android Kodi and any sort of deinterlacing.

You HAVE to use LibreELEC Kodi Krypton, and that cannot run on a Mi Box.
You bought the wrong box if you want to use high quality reliable TV deinterlacing.