Hi there! As ExpiredPopsicle mentions in his PR, this is caused by some erroneous assumptions in the scene outliner code that attempts to remove dead actors from the outliner tree.
There has been significant effort for the upcoming 4.7 release on the Scene Outliner, and we hope that it now functions more responsively and robustly, particularly when dealing with the sorts of issues as are being raised here.
To test this thoroughly I’ve made a level that spawns many hundreds of actors and deletes them both instantly, and after a delay. Whilst I see some short-lived (Deleted Actor) entries, they never persist past the regular scene outliner updates (it updates every second or so during PIE).
Of course, if this continues to persist, even in 4.7, please let us know so we can investigate further.