My T Mobile sim unlocked phone get network locked after 4 years

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Nov 1, 2014
Hi guys

I have a S8+ (G955U)which one of my friends got it for me from US with permanent unlocked network . i've been using the phone perfectly for about 4 years .recently i gave my phone to another person and before that i did reset factory and wipe data and cache in boot menu everything was good but after 2 days the phone got network locked and needs PUK network code which i haven't and as i don't live in Us and have not Tmobile sim card , i wasn't able to contact T mobile customer support to adk why this happened Also The official Tmobile Unlock apps wont work on my phone first i thought its because of My Country (i live in Iran and everything is boycotted here!) but after using diffrent VPN the problem didn't solved. I am pretty sure the phone was legally network unlocked and there is nothing suspicious about it but now the phone needs the code which I haven't at first place.

Does anyone can Help me find out why this happened to my phone?