Causing a Nav Mesh update, such as by moving an actor into the NavMesh that will create a hole, seems to be breaking Nav Link Proxies by preventing characters from being able to use them.
Result: When MoveableActor affects the NavMesh and causes the hole to appear, the character is no longer able to use the Nav Link
Expected: The object affecting the NavMesh would not break the Nav Link and the character would be able to move back and forth as it would if MovableActor was not in the scene.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-41884 in the post.