The issue is a race condition that only reproduces under the following conditions within a frame:
This results in a crash on the mobile base pass mesh processor, which attempts to dereference the cached reflection proxy. AddToScene will attempt to cache the reflection proxy, but only if it is null. Thus, cached proxies must be nulled out after a removal event to avoid this scenario.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-77001 in the post.