Magic projectiles don't disappear with the caster
When a summoned creature disappears in Morrowind, all projectiles they fired as well as magic effects active on others caused by the summon will instantly disappear. Did a bit of testing and found OpenMW already removes the magic effects, but not projectiles that are mid-air.
Easy to test with an item enchanted with a constant effect Flame Atronach. When you unequip it, projectiles it fired will disappear with the summon.