For a lack of a better place to post:
I can import XF-AVC footage from a C300mkii or C500 mkii into FCP without problems.
But the clips do not play properly in Quicktime, either in the MXF or MOV version?
VLC plays them fine.
I think that was working at some point in the past but not sure.
Any ideas where there could be a problem? Maybe a software conflict somewhere?
VLC uses completely self-contained, reverse-engineered media support. By contrast starting with Catalina, Quicktime Player is dependent on formal OS support using the MacOS AV Foundation framework.
I think this is the situation: In MacOS versions before Catalina, Quicktime Player and Quick Look formerly worked for a broader variety of codecs due to the 32-bit Quicktime 7 *framework* having system-wide 3rd-party codec support. IOW a vendor such as Canon could supply a workflow package which after installation would not only work for their utilities, but would extend that codec support system wide. Other apps and utilities such as Quicktime Player or NLEs which called the Quicktime 7 framework would get that support. Starting in Catalina, that went away.
Often described as a move to "64 bit codecs only", it was really a move from the 32-bit Quicktime 7 framework to the 64-bit AV Foundation framework (which does not have documented 3rd-party system-wide codec support).
This creates a situation where individual apps can build in codec support using AV Foundation framework, but it's not system wide. E.g, FCP has support for Sony XAVC-I but it won't play in Finder or Quicktime Player. You are probably seeing that same behavior with XF-AVC from the Canon C-series. Yes it is frustrating. You could try the latest Canon workflow package.
In fact there is apparently undocumented 3rd-party system wide codec support in AV Foundation, because Avid DNxHD is now supported, whereas the original Apple article said it would not be: support.apple.com/en-us/HT209000
I don't think Apple writes that codec support, it is probably written by Avid then via an undocumented process Apple integrates it in MacOS.
It's possible that dropping 3rd-party codec extensions was done due issues of reliability, performance and support. It's easy to get into the "codec pack nightmare" where some junky codecs were installed then that causes problems. It could also have caused problems on Apple Silicon.
But I can't play any Sony MXF files from the FX6 in Quicktime/Quicklook, and several codecs from the A7SIII/FX3 don't play either. They all work in FCP but I have to import them view them with VLC (which is clunky) or use Kyno, which works but is $160: lesspain.software/kyno/
Thanks so much Joe for the detailed reply. Makes a lot of sense (unfortunately)
Have already installed the Canon XF utility 3.8 package but as you guessed it doesn't change anything.
The main problem is that the audio playback for the XF-AVC clips is out of sync in AVFoundation (playback via Quickview or Quicktime or apps using that framework). And that is the same whether they are in the MXF or MOV wrapper.