Welcome to the forum!
Sorry you’re having problems.
The threads where OS15 is suspected of being a problem have been tagged as “researching”. The devs are actively investigating this but when it’s not an issue that affects everyone they have to first try and duplicate the problems and then solve them.
Another variable that the devs can’t control is if Apple releases a patch for OS15 that may solve the problem it introduced in 15. It can be a “moving target” after a major release of the OS. That along with the multitude of different AV equipment that users have and combine differently may make some problems hard to duplicate.
I too and not seeing any issues with HDR or frame rate matching and playback for me has been problem free.
As @bhampster said, if you’re having specific problems it would help to have a bit more info on what you’re experiencing and what type of files (resolution, container, etc) you’re seeing this on. Also what device/model as well as connection protocol and method (WiFi or Ethernet) may help.