When a chunk loads, the mobs on that chunk spawns in the same place and it looks really weird.
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It's a pretty irrelevant "bug", It's hard to see and I think it work as intended, but on slow PCs like mine and in minimal render distance it's more visible and looks very weird.
I am unable to reproduce this issue in 20w49a. After loading a numerous amount of new chunks, mobs always seemed to spawn within groups, but not actually inside of one another. The only incident where this sometimes happens is when a witch and cat spawn in the same location, inside of a witch's hut. See MC-163503.