The bug

The resin_clump's back texture is mapped "forwards" on both sides, causing some pixels to not appear in the same place when viewed from behind.

How to fix

The cheap and quick fix would be to simply reverse the mapping for the resin_clump model. However, this would be ignoring a bigger problem: despite there being multiple different "flat plane covering a block face" blocks in the game, they all use their own model files, rather than referencing a central template model like cubes, crosses and so on do.

So to fix this, I'd strongly recommend creating a dedicated "flat plane" template model, and having ladders, glow lichen, sculk veins, mossy carpet side units and resin clumps all point to it instead.

Non-sloped rails could also be made to use such a system (MC-212626), and lily pads and vines could also point to a tinted such template model.

The absence of a central model has caused issues before with resource packs and mods, so introducing such a model would make things much easier.

Doing this would also fix MC-249079 for free.