Pixelated image playing original quality, but not when transcoding

Server Version#: Version 1.41.2.9200
Player Version#: unsure, but likely current

Firestick 4k Max (hardwired - tests at 300-400Mbps)
Server N100 Ubuntu, USB drives

When I play some episodes, I get pixelation that comes and goes when I am playing original quality, but is perfectly clear if I swtich to transcoding. I recently upgraded to a 4k tv, and was afraid it was just something I had to live with when upscaling (in this case, 480 to 4k). It is only in certain episodes and always at the same places in those episodes. Also happens on my Android tablet. Does not happen in web player, Linux desktop player, or android phone. Does not happen in VLC. In the example below, the pixelation comes and goes throughout the scene, usually lasting less that a second each time.

With the two shows where I’ve noticed it (Flight of the Conchords and Parks and Rec), it’s only a problem on the first couple of episodes, then never again throughout the series. All episodes ripped from commercial disks with the same settings and no post rip processing.

It seems worse in images with lots of parallel lines like that fence. Could that be causing a glitch? But then why wouldn’t it also affect transcoded versions…

Thanks for any ideas.
Mike

Also seeing this on a Firestick 4k Max. No problems with any other content. The pixelating is sometimes with different content. Only noticed it on non-HD content recorded by Plex (Freeview UK).