Reproduced 6/6 times.
This seems to affect all "Set" functions for Geometry Collection Components, such as Setting Object Type from Static to Dynamic, or changing Collision Type.
Actual Result: The Geometry Collection maintains its shape
Expected Result: The parts of the GC not inside Anchor Field should fall apart
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-150877 in the post.