I just got off the phone with Pro Apps support. I reproduced the problem while they remotely viewed it. We created a new user account to ensure it's not user-specific (it's not). I uploaded to them various diagnostic info they requested.
I previously tried to reproduce it with some GH5 HEVC test material but could not. Others have reported problems using 10-bit HEVC4k material from the EVA-1. I'd like to get some similar test material from an S1 or S1H since those are similar. One person in our doc team has an S1 so I will ask him. Both of my data sets are UHD 4k 8-bit 4:2:0 at 23.98, although the Arlo camera's frame rate varies. The X5S camera on the Inspire 2 is fixed at 23.98. All my machines are still on Mojave, but a few people have reported similar problems on Catalina, so it currently does not seem dependent on macOS version.
Given the right data set the problem is 100% reproducible but it seems to require several files. It can take up to a few minutes to happen. I will try to narrow the replication scenario in case Pro Apps needs it uploaded.
My gut feel is it might be related to the Metal optimizations in conjunction with certain HEVC material.
The administrator has disabled public write access.