iMac 2020, 3.6 GHz 10-Core Intel Core i9 with AMD Radeon Pro 5700.
That format is not supported for hardware decoding with your CPU; sorry about that. See this article by Puget Systems. I recommend transcoding or using proxies for this format when paired with that iMac.
Sorry for the experience. I'm in support and just showing you an article that I thought might help you understand what's currently being seen by other professional users. My advice is as an editor and what I would do to workaround the issue (I am a big fan of editing codecs). I am not on the dev team and can only make them aware of your issue.
I hope the team can restore this ability for you and others trying to use this format, which is difficult for any computer to handle. Right now, as you can see from the matrix, only the very latest Intel CPUs support it.
I understand your frustration and would be happy to advocate for this capability to be available in Premiere Pro.