highly recommend getting systems to latest 15.1.x maintenance version at minimum. Might require a multi-step upgrade depending on where you are now, reference AskF5 Solution K13845 for details.
The only open bug I could find against 15.x is this one, but it doesn't seem related. Post back your specific v15.x version, a sanitized version of your istats collection configuration, and I can run a test on my local lab. Would also recommend opening a case to have official support weigh in.
