Earlier quoted context omitted.
This Reddit thread concluded that because a T-mobile account was suspended for non-payment and the number is unportable while suspended that the customer was out of luck. https://www.reddit.com/r/tmobile/comments/4wotw8/tmobile_won... I don't see how that matters legally. The FCC requirement doesn't make portability conditional on some account status defined by the carrier. But IANAL.
Since T-Mobile holds the primary numbers for Google Fi accounts, going from T-Mobile to Fi legally isn't a port, just a change in T-Mobiles billing system.
If your port is delayed or denied, file a complaint with the FCC, as this starts the regulatory review clock ticking.