de0u I have contacted google support and have chatted with multiple representatives. The first representative offered to replace my phone without any hesitation, making me think that the issue is very hard to solve. And another one asked me to send the phone to somewhere to repair it. But because I travelled shortly after I got the phone, I'm not in the US, so I can do neither of the two options. At last another representative submitted my case to their engineer for further investigation, but I haven't got a reply after several days.
I searched through all platforms discussing Pixel phones, and I only find one person on XDA who has a Pixel 10 Pro XL with Google Fi has the same problem as me. My own theory is that because Google Fi sim has a T-mobile sim profile actually, Google server thought it was a T-mobile phone for some reason. I heard for older Pixels. if you activate a US carrier on a UNLOCKED pixel before you look into developer options and make sure oem is toggable, it might be grayed out.
Again, I would recommend turn on developer options first and make sure OEM unlocking is toggable, and activate any US carrier later.
I'm a long long time google phones user but I never met such a issue before. Nexus and Pixel purchased directly from Google were always unlocked. But this time, I really don't know how to solve the problem now. Any help will be appreciated!