Blueprints reverting all fields to default after compile

I recently migrated a project from 4.16 to 4.18 and started having this strange problem.

Every time I add/remove a function or a property to the header of a class, the child blueprint of that class will lose all its references and all its values will revert to default after compiling.

The child blueprints have no nodes in them.

Any idea what’s causing this ?

Edit : I just found out about an important distinction. The problem only occurs if I add/remove a UFUNCTION or a UPROPERTY.