I'm not too sure if this affects more than just 1.21, but movement while... I'm not sure, just movement causes weird texture glitches, I guess?
Not just on moving blocks, also on regular grass and whatnot. It's very confusing but it seems to take the textures of the blocks around it, for example here it's a tnt duplication machine, but it also takes the textures of things like water, I believe. It's very quick so in order to replicate it, I'm not sure how one would, but I had to record videos to get it to capture because my reflexes were too slow.