What i expected: The trims would combine or that the smithing table not allowing to put the already trimmed armor in the slot
What happened: The armor is allowed to be placed in the armor slot, but it deletes the trim, that was there before.
How to recreate: Make an armor piece, put a trim on it with a mineral. After taking out the armor piece put it back with a different trim and a different mineral.
this seems like intended functionality, like dying a colored shulker box a different color. If you want to change your trim you can but you cant have two.