We had an issue on particular appliance were all log files under /opt/mwg/logs/user-defined/access.log/ folder were not rotating daily as per time set under Configuration > Log File Manager > Enable Scheduling of Log File Rotation.
Instead the access.log files would rotate when the specified maximum file.size was reached.
Turns out that if a foriegn file exists in the affected folder (ie. not access.log) then the time based rotation will fail. Solution is easy - delete the offending file.