Ok, I found out what's going on, but this is sort of embarrassing ...
It turns out, I have been ignoring a nice little warning regarding the use of Vector as type of a field that holds a @xxToMany relationship with FetchType.LAZY:
Element [field someField] within entity class [class Campaign] uses a collection type [class java.util.Vector] when the JPA specification only supports java.util.Collection, java.util.Set, java.util.List, or java.util.Map. This type is supported with eager loading; using lazy loading with this collection type requires additional configuration and an IndirectContainer implementation that extends [class java.util.Vector] or setting the mapping to use basic indirection and the type to be ValueholderInterface.
Two possible solutions can fix my behaviour:
* use FetchType.EAGER (then I could stay with Vector)
* use List (as the spec says...)
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