With Bytecode enhanced Immutable Entities referenced cached in the 2lc, a reference is retained to the EntityEntry, and in turn a StatefulPersistenceContext, associated with the Entity when it was loaded and stored in the 2lc. The PersitenceContext (and related session objects) are not GC'd after the session is closed.

When there is a high cache miss rate a lot of sessions create cached entities, a lot of sessions are retained in memory