One of current annoying limitations of Blender re Collections/Objects is that objects are forbidden to not be instantiated in at least one collection. Code ensuring that as a pst-processing step of override creation/resync operations would be a bit too eager to add those objects to an external 'ad-hoc' collection, which poses several issues (both in term of keeping the scene well organized, and related to override hierarchy handling). So now be very conservative and only generate and use external 'storage' collection for those objects when it is absolutely mandatory. In pratice, it means this should never happen anymore on any decently organized data source.