StatefulTreeCache.get() is passing the wrong Fqn to region.markNodeInUse:

// Mark the Fqn telling the eviction thread not to passivate it yet.
// Note the Fqn we use is relative to the region!
region.markNodeCurrentlyInUse(new Fqn(key.toString()), MarkInUseWaitTime);

It should be

// Mark the Fqn telling the eviction thread not to passivate it yet.
// Note the Fqn we use is relative to the region!
region.markNodeCurrentlyInUse(getFqn(id, true), MarkInUseWaitTime);

otherwise the extra "bucket" level in the Fqn is missing.