In general, items you can buy from NPC vendors and which are not event-locked items, cannot be stored in the Armoire and are not intended to be storable there.
What can be stored in the Armoire are one-off items that you usually cannot buy more of.

Most of the so-called artifact armor is indeed not storable in the armoire. Nor is it supposed to be.


There can of course be a few mistakes about what can and cannot be stored in the Armoire, but overall I dare say it is working as intended.