The top surfaces of some blocks often fail to render properly when the player is both located on an integer Z coordinate and facing directly south.
Steps to reproduce
To align yourself properly, you can use /tp @s ~ ~ z.0 0 0, where z is your z coordinate. Note that omitting ".0" implies ".5", which will not work!
Position youself such that a block's top surface is visible directly next to you. Make sure not to move forwards or backwards or rotate the camera.
The glitch is easier to see on high FOV, like in the video, but is more subtly visible even on normal FOV.
Apologies for the video quality; had to fit it in the 10 MB limit.
edit: Added a H.264 version of the video since at least my browser seems not to like the H.265 one. Even more apologies for the quality there.
Update: I was unable to reproduce this on Mesa's software driver; this may be an AMD driver bug.
I can't reproduce this issue in 1.21.4 pre-release 1 on an Intel igpu. Maybe you should report the issue to Mesa developers https://gitlab.freedesktop.org/mesa/mesa ? Oh, and the correct command to align yourself is /tp @s ~ ~ z.0 0 0 not /tp ~ ~ z.0 0 0 🙂