I have experienced the same thing. Modem port forwarding is working fine when setup in Oct 2017.
Then it happened once on 20 Dec 2017. All except one port forwarding settings were all gone. So I setup again, and everything is working fine until 12 Jan 2018. It happened again. So I setup the entire port forwarding map again. Today 24 Jan 2018, I lose them again!!!
Whenever this happens, it appears that the static IP address setting inside DHCP reservation table is missing from the original ones. I know because I renamed them every time I reconfigure the lost port forwarding settings.
My Huawei HG659 reboots everyday since day 1.
So obviously something is wrong with this modem. All settings should have been stored in the EEPROM, but clearly it is one hell of an unstable modem, and something is faulty inside.
TPG must fix this issue with Huawei.
Presently I am waiting for Modem replacement from TPG.