[damagedspline]

There seems to be more than just a runtime incompatibility. I have a Ryzen 2400G and your Matrix build (KodiSetup-20210601-e5b3fd32-19.1-Matrix-mvc-x64.exe) didn't run on my system while the latest Leia MVC build (KodiSetup-18.9-mvc.v18-x64.exe) runs perfectly with software MVC decoding.
Windows 21H1
Latest AMD graphics drivers 21.9.2
libmfxsw64.dll version 7.16.5.13

To make sure that VC runtime incompatibilities aren't the root cause I compiled your mvc.v18 branch and the xbmc Matrix branch with your two mvc patches applied with VS2017. Both self compiled versions show exactly the same behavior as the builds provided by yourself in this thread.

Leia MVC works perfectly with software MVC encoding and interlaced, sbs or ou output.
Matrix MVC build, setting "hw mvc decoder support level" < 20: The video starts, but the screen stays black. The OSD is correctly shown in 3D. The log is flooded with the following error messages as long as the video is running:
2021-09-24 16:21:01.762 T:9156    ERROR <general>: DXVA::CProcessorHD::GetInputView: cannot create processor input view.
2021-09-24 16:21:01.762 T:9156    ERROR <general>: Skipped 1 duplicate messages..
2021-09-24 16:21:01.762 T:9156    ERROR <general>: DXVA::CProcessorHD::Render: incomplete views set.
Matrix MVC build, setting "hw mvc decoder support level" >= 20: The video is visible, but only in 2D because of:
2021-09-24 16:21:35.409 T:6112    ERROR <general>: CMFXCodec::Init: MSDK not available
Full log: https://paste.kodi.tv/hoqiviruzi.kodi

I know that sw mvc decoding isn't a priority here and  I highly appreciate your efforts to keep 3D for Kodi alive. But maybe the info helps to keep it alive also for AMD systems.

According to what you describe mfx (>20) is able to parse the 3d file but fails to render it.

This might be related to my current issue. My new intel box which arrived today (j4105 cpu) is able to switch to 3d view on the hdmi2.0 port but only one eye is showing the movie where the other is either blank or the rotating progress or the windows background so at least windows os can go full 3d mode. Sadly, remote debugging is crashing kodi so i cant trace it remotely. As a result, i am now putting a full dev env for kodi on this tiny device to attempt to trace the issue.as you noted dxva rendering is related to both our cases.