All armor trims can be put on armor, but don't appear.

This happens in all of the RPG Series, not just in paladins. I will only report this here for now, but i could report on the other mods' pages if needed.

This is not a bug.
Trims are not supported, as the custom armor models have a mismatching geometry.
It would take lots and lots of labor to fix this, hence the decision is that the armor trimming mechanic stay a vanilla gimmick within the scope of the RPG Series.