Greetings;
GUESS: ZOOMING ISSUE, NOT A BUG; (alias moire)
While I do not posses the utmost knowledge on the internals, this looks a lot more like a zooming artifact to me.
Consider zooming in and out? Does it follow the pattern only for the specific zoom?
If so, it is a famous anti-aliasing artifact
It is known as moire pattern
[wiki] in typography;
p.s. Hmm, a better link would be
aliasing wiki page it even showcases the problem in the top picture.
Off-topic part:
In Factorio you can often see parts of the picture on the screen updated in different chunks on a seemingly "random" pattern.
Easy example would be map-thumbnail updates when the tile set_tiles is run each tick in a rainbow. If the left few reds are on the chunk or render-chunk border it might be the reason. But uneven dark green lines in the picture suggest a zooming artifact.
Even if small "unfair" pixels could be substituted to the expected-average-color instead to combat the "aliasing lense", there is little point to do that. A better take would be to make thumbnails (like a few x few pixels) for smaller zoom levels and render them instead (bigger error, but more "stable" pic and possibly less pix to bitblt').
But even that is questionable since it consumes additional chunks of gpu-mem which comes at a linear small yet leap price with rainbowish belts of modded stuff.
Personally though, I do not even CARE about each item on the belt THAT much.
If it would, say, change every 4 items of same type to a single one + bg-fill it would even be easier to look at and easier to spot full belts.
But then mixed-ore setups from scenarios come in. And any additional fps drop to belt items is not worth it! We already have huuuge drops for medium-to-big blups when they are rendered (full res!) in the cursor (moderate arty train on rails is a simple testcase), so let's just spare belts as they are. It works.
Good luck :>