The three Dunmer ghosts (dunTolvaldsCaveGhost01, dunTolvaldsCaveGhost02, dunTolvaldsCaveGhost03) in Tolvald's Crossing turn into ash piles upon death. It seems like they should either just disappear at that point or turn into a pile of ectoplasm. There were similar issues with ghostly NPCs in Forelhost and if I think they were modified to vanish when killed. Not sure if you think it's worth bothering with these three NPCs though since they're a one-time encounter.
From a previous ticket (MadCat's quote):
Ashpiles are what the game seems to revert to as default when there is no designated remains pile mesh to use. The fake Seeker daedra in Dragonborn DLC had a similar problem; the dev who set them up didn't realize that leaving a null value for the remains pile doesn't make it do nothing. They were fixed to have a "if ashpile != 0" conditional on it to prevent it from doing anything.