At a licensee, I discovered a problem where Blueprint-created array and map properties are not correctly propagating the ContainsInstancedReferenced property flag. This means that if you create a Map or Array containing references to DefaultToInstanced classes like UWidget or UActorComponent, those references may not get get correctly set to the instanced widget/component when an actor or widget is instanced into a running world. The instancing seems to work correctly in some cases without those flags, but it definitely fails when spawning widgets in Cooked builds as they use a slightly different instancing path than in Editor builds.
The fix is to correctly propagate the ContainsInstancedReference flag for BP-created properties, the way it was already working for native properties
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-74204 in the post.