Non-instanced object ref properties are incorrectly being skipped at property initialization time when Blueprint class instances are spawned with the fast property initialization path enabled.
This results in a subobject instance that's unique per Blueprint class instance, so it behaves as if it were instanced. It should instead be referencing the subobject instance from the default data (parent CDO), in order to match what happens when the fast path cannot be taken (e.g. when spawning child actor components).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-191334 in the post.