Hm, unfortunately it didn’t change anything. I tried reproducing the problem in a new project, but was unable to do so.
I tried creating a new level with nothing in it, setting the GameInstance to the default GameInstance class, and using the default GameMode class. The (deleted actor) appears immediately upon hitting the Play as Standalone Game button (and Play In Editor/Simulate still doesn’t have the problem), before the little splash screen even appears. Maybe my project is just corrupted somehow.