I have been encountering this issue since 1.16, where while modding using community modloaders I found out that my own custom entities that are smaller than 1 block would have issues jumping over some blocks.

I attempted replicating this issue in Vanilla Minecraft today, and would you look at that: the issue is there still with any mob who's collision box is smaller than 1 block.

If a small mob tries jumping on top of a block, and there is air in front of said block, the mob will get stuck and keep running into it without jumping.

I have shown in the videos how to replicate the bug, a structure like shown should be enough to replicate it. Have a mob either run away from you or chase you, and the issue should appear.