Users are removed from Group Set

Dear All,

I have a strange issue with my TMG server, as some of the users are removed automatically/randomly from a group set. This happened 6 times last two month.

TMG Setup:

I have 2 TMGs servers installed in DMZ network and running in single Network adapter, I’m using TMG for web application publishing only, Active Directory is installed in other network Layer with no restriction between TMG and AD.