Forget this network for iPad and Personal Hotspot OFF for iPhone do not persist? Need repeat set both devices?

How to prevent second device permanently NOT auto join iPhone hotspot which was already set as “Forget this network”?

I already turn off Personal Hotspot for iPhone which has limited cellular mobile data.

after I unplugged SIM from my cellular iPad, and I set it to forget iPhone network, iPad only forget temporarily but reconnects; my iPhone cellular mobile data has been used up without my knowledge.

I suspected combination of Force Restart and iPhone disconnect from WiFi is cause.