Business Rules suddenly go haywire -> @LIST with @UDA behavior has changed?

Summary:

The FIXes in my business rules suddenly resolve differently due to a change in behavior when using @LIST(@UDA(Dim,UDA))

Content (please ensure you mask any confidential information):

I'm using Set EMPTYMEMBERSETS ON with the following FIX expression:

FIX(@RELATIVE("EntityParent",0) AND @LIST(@UDA(Entity,"UDAx")))

FIX(...)

Command;

ENDFIX

ENDFIX

Until recently, this returned a list of lvl-0 entities below that "EntityParent" that had UDAx assigned. Now, this suddenly returns ALL lvl-0 entities below "EntityParent". As you can imagine, that is a bit of a problem.

When I change the outer FIX into this, it works like before/expected: