Glad you’re so appreciative and worked through it! I gladly share, discuss, and respond.

I’ll have to read up on palette filters. :) I do semi-regularly use ffmpeg, but palette filters are not something I have heard or used before.

I assume in this case it’s a downsampling into fewer colors, evading the issues of almost-same-colors?

Especially given the last square/check pattern makes me thing of codecs splitting into square blocks and then encoding those. It could make sense that this division leads to different results for one reason or another, which then produces a check pattern without it being there before.