Nighthawk losing Port Forwarding configurations

Hi,

I've owned my Nighthawk R7000P for 3 years now.

I have been port forwarding a few ports with no issues at all, but after a while I notice when I go to view the ports that are currently open in the "Port Fowarding / Port Triggering" page, I find that there are no listings at all. Almost as if the router has completely forgotten the configurations?

I have tested the ports using a Open Port Checker Tool and they are closed. 

The only way to fix the issue is to revert to a configuration backup file I have saved. Then after a few weeks, the same problem occurs again.

I am running Firmware Version: V1.3.3.154_10.1.86 (Latest I believe)

Any help would be greatly appreciated. 

Thanks