A brief description on the mechanics of this issue and the solution for it.

The 'nulling' of foreign keys is called a cascading action, in particular the {{ CollectionRemoveAction }} was not being called in this case. {{ CollectionRemoveAction }} are created on {{ AbstractFlushingEventListener }} where it iterates over a list {{ CollectionEntry }} in the {{ PersistenceContext }}. For collections not initialized yet there was no {{ CollectionEntry }} and therefore their keys were not being 'nulled'.

{{ CollectionEntry }} are added as part of the load process, and in particular in the type resolution of {{ CollectionType }}. For lazy fields, type resolution is always skipped on load because the value that results from that process is not needed. Resolution can occur on later if there is a lazy loading. This clearly has to change, so that type resolution is always done for collections.

There is still one difference between resolving a lazy and a eager collection. In the lazy case, the {{ PersistentCollection }} must not be added to the 'hydratedState' of the entity or else the collection will be loaded when the attribute values are set on the entity object.