Ranir Domov level 8 Druid
I summon my Water Elemental Companion and then use an item to conjure a Water Elemental summon, when the summon appears it immediately dismisses my companion.
As it uses kind of summoning system, items will break it. Items use old summoning style, mostly, where they replace every other summons for a single one.
Use the item first, then call your animal companion. Same thing will happen to all summons when you use an item afterwards.