FS#21865 - Dead thralls disappear yet Reanimate Thrall remains as an active effect.
To expand on the summary, thralls summoned with the “Dead Thrall” spell will occasionally disappear for an unknown reason. The active effect “Reanimate Thrall” persists and currently the only way I’ve been able to remove it is by going to jail. Cloak spells used to fix it on the 360 version, but those aren’t working for me. Having this effect active all the time causes NPCs to comment on your “dangerous spell” rather than standard greetings.