Spoke too soon. 
The issue persists. Making the changes above made it work temporarily, but now the issue is back.
TOO MUCH TIME SPENT ON THIS with no progress.
By the way, in parallel, yesterday midday or so, I upgraded Shield Experience from 8.2.3 to 9.0.2. It had no difference one way or the other on the freezing for these UHD HDR AC3 5.1 clips.
To reiterate the video freezing (audio continues) always happens at exactly 2 minutes 48 seconds after stating the playback form the beginning.
This is happening on 2 different TVs with Roku Ultras attached and happening with.at least 3 different clips at 2 minutes 48 seconds.
Really puzzling!
I will try the Roku 4K stick when I get a chance and see how that works (may try the Chromecast 4K; picked both today).
