When using Vanilla Tweaks' Directional Hoppers resource pack, the wooden hopper's sides take on the texture of the resource pack without changing the colour.

This issue also presumably applies to other resource packs that change the texture of hoppers, though I have not tried this.

Screenshots:

Without resource pack:

With resource pack:

I'm encountering something similar on NeoForge. I'm guessing this is also a mod compatibility bug but I'm not sure which mod it would be.