Armor with trim cannot be disenchanted with grindstone

While also having this issue, I discovered the item with trim can be disenchanted when placed in the lower slot on left side of the grindstone gui.

After testing with a diamond helmet with netherite host armor trim enchanted with Projectile Protection I, It could be disenchanted on the bottom slot but not the top, however, after putting the enchanted diamond helmet in the top slot and a second base diamond helmet in the bottom I could disenchant it

Confirmed in 1.21.20.

You may want to update this ticket to explain that it works fine when in the bottom slot but won't accept it in the top slot unless another type of the same armor is in the bottom slot.