Was testing a little today:
I hooked up my HTPC using a Display Port to HDMI cable that only supports up to 1080p.
I was then able to use MC23 to playback 3D in windowed mode (still does not kick into FSE even though selected).
I then swapped cables back to HDMI 2.0 and was able to set the desktop res to 4K/60.
From that point MC23 continued to playback 3D properly.
If I rebooted any device in the HDMI chain -- HTPC, Marantz AVR, LG OLED TV, then MC 23 no longer would output 3D correctly.
Again, with MC22, works fine every time including kicking into FSE. Same version of madVr in each case as I updated this for both MC22 and 23.
Still think this is an MC issue; curious what others discover
I at least now know that it was the addition of a 4K display into my chain and move to MC23 where the trouble began
I may keep a 1.4 cable connected to another HDMI input on the Marantz and 2.0 cable connected for 4K content and see if I can just swap between them when I watch 3D movies.
It's not the most convenient, but it can at least be done from the comfort of a chair