Stateful Session Beans not garbage-collected after being removed from cache

We're experiencing a problem with our app on WLS 12.1.1 where stateful sessions beans are not garbage collected after they are removed from SFSB cache. If we set cache-type to NRU and monitor the EJB in WLS console, the beans are never removed from cache. If we set cache-type to LRU, we see the Cached Beans Current Count tick down to 0 as each bean ages to the configured timeout value. However, when the cached bean count is 0, jmap and visualvm show that none of the bean instances were garbage collected, even after we force GC with WLS console or visualvm. The visualvm