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All the animations in the belts have been replaced by two layers. The bottom layer is the grey belt. The top layer is a tinted arrow. It works perfectly well for belts and for splitters. But with the other two types, odd garbled artifacts occurs for the top layer.
I tested and tested and it's something to do with the small size of the arrow and possibly the fact that in the starting_* and ending_* animations, some of the frames are completely empty. If you have two layers of equal size and with pixels in every frame, it starts working perfectly for all four entity types. But I can't explain why it works perfectly well with the splitters, and not the other two layered types. When I look at the atlas sheets by pressing F3, the tiny arrows are all cropped and all mashed together with other things. That's not a problem for the belt and splitters, but it seems to be a problem for the undergrounds and loaders. If I go into graphics options and change the atlas size, I get different garbles.
I'm calling this a bug because it's (a) odd and (b) annoying, YMMV.