Compile errors caused by an actor calling a function with something hooked up to a removed parameter can cause that actor to lose it's place in the hierarchy. Typically, the actor will be fixed once the node is fixed up and the blueprint successfully compiles. However, if the level is saved while it's in that state and checked into source control, the actor data can be permanently lost.
Linked UDN provides a video and example project.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-64816 in the post.