> I learned that Google Fi exclusively uses the T-Mobile network for iPhone customers, and activating a Google Fi SIM with an iPhone causes T-Mobile to lock the phone to the T-Mobile network (which is illegal, since the user hasn’t entered into any service agreement with T-Mobile).

This happened to me with an unlocked iPhone, bought at Best Buy, that I activated with a T-Mobile SIM card as well. Tried Apple support first, Apple couldn't care less. In the end T-Mobile themselves unlocked it after 2 full months of calling them almost daily and escalating, reaching out on social media, etc. This has nothing to do with Google Fi, and, if anything, I blame this on Apple for such a stupid fucking design that lets carriers unilaterally lock a brand new phone with no recourse.