When turning or moving fast at certain heights, blocks will clip through Iron Bars above them. This is most prominently seen with Quartz Blocks, though can be seen in with other blocks as well.
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Can confirm that this is an issue in 20w51a.
Can confirm in 21w06a.
This actually appears to be a duplicate of MC-192420, since this has ticket has a "Mojang Priority" attached to it.
Affects 20w21a. Can I request ownership?