[clarkss12]

So does that mean that playback of 480i, 576i and 1080i (three of the four most common standards for OTA/terrestrial, satellite and cable TV - with 720p being the other) are non-starters for frame rate switching in Kodi with this box?  If so - that's a real red flag for anyone who uses Kodi with a PVR back-end (TV Headend, VDR etc.)

(Is it just interlaced modes that are problematic - or is 720p also an issue?)

Only the interlaced videos have that issue, NOT the 720p media.  So, Yes, I have to disable that feature when watching media broadcast with those interlaced media.  I have been providing Mecool with information on the issues their firmware has.   I also have the HDHomeRun Connect 4k tuner, and those HEVC channels don't have that issue.

The KM6 Deluxe ALWAYS displayed "HDR 10" under my TV's stats for nerds. no matter what media I was playing.  I emailed them about that issue, and they corrected it.  A couple of days ago, they sent me a firmware to test, and it solved that issue.

just for clarification here - does the updated firmware actually fix the always on HDR output or just fix the display showing HDR10 regardless of media encoding?

I don't have a way of confirming it, since I no longer have a 4k TV that does NOT support HDR.  I can say that when playing a NON HDR video my TV stats does NOT show HDR.  When I play an HDR10 video, my TV stats show HDR 10.