One update to this, is I've discovered this is not iPhone 12 specific, as the same animation behavior occurs in the iPhone XS, accompanied by the same RunningBoard errors.

I have 2 feedback reports for this: FB8903850 was submitted back in November when I was on the iOS Public Beta releases. This includes a screen recording and sysdiagnose. FB8993381 I submitted today, running iOS 14.4 back on the general non-beta releases. This includes 2 screen recordings. I was not prompted to install a debug profile for sysdiagnose data, perhaps because I submitted this feedback through my laptop rather than my phone/app?