For example, if I try to write 5500FF, which is a valid hex code, it will only let me type 5500. I have only tried this with 1.18.2 and Fabric so far, but it may be happening for other versions and modloaders.

The 1.18.2 version doesn't use hex colors, but integer colors instead.
This was changed in the 1.19 version, but I don't plan on backporting anything to outdated versions of Minecraft.