I'm not familiar with the networks in question, but I've run into a couple wifi networks connected to my lte carrier, and my unlocked phone pushes them hard. I imagine there's something coming from the SIM, and I could see the OS picking up SIM affiliated wifi networks on boot or sim insertion. Unless special care were taken, it makes sense that forgetting a network isn't remembered.
As far as I can tell, it's because the configuration comes from EAP-SIM (https://datatracker.ietf.org/doc/html/RFC4186) which gets refreshed on every reboot. This cannot be controlled - it will always regenerate when you restart the phone.
That seems odd. My phone doesn't connect to a network unless it's toggled to auto join, and certainly not post "forget". Can you not control this?