Weird issue in security cache

Hi XWiki devs,

We ran into a weird issue on XWiki security cache. Our entire security cache is purged every 4 hours.

Some background about our setup:
We retrieve users’ group membership from LDAP
We set a 4 hour expiration time on the cache(as below) because LDAP records may update at any time.

<local-cache name="platform.security.authorization.cache">
<eviction max-entries="50000" strategy="LRU"/>
<!-- 4 hours -->
<expiration lifespan="14400000"/>
</local-cache>

To our understanding, each cache entry should have its own timer which expires after 4 hours.
But what we observe from JMX dashboard is that the entire security cache is being purged every 4 hours.

In the attachments, the Security Cache.png is security cache JMX diagram, the Group Cache.png is our user group which has similar issue but purge irregularly.

This problem is cause big performance issue in our system. We can’t figure out the root case. Any pointer will be highly appreciated.