After some debugging, i reduced it to AbstractTypeBeanResolver#resolve where the beans will be copied.

In Version 2.4.1 the type of the incoming Set (beans) is an ImmutableHashSet therefore the optimized copy routine of ImmutableSet takes place and just returns the Set.

In Version 3.0.0 the type of the incoming Set is an java.util.Collections.UnmodifiableSet and so the Set is copied every time.