Honestly, Google Pay has a pretty bad habit of forcing re-authentication despite having already done so to unlock the phone--regardless of merchant (and whether one uses an 'approved' unlock such as fingerprint/PIN). This might possibly be US-specific as EU phones/cards seem to be allowed to tap without phone unlock up to the country's contactless limit.

Of course, considering how contactless seems to be implemented by merchants here, I wouldn't be surprised if the contactless limit in the US is effectively $0 (but unlimited in practice due to signature being requested for physical cards and signatures mostly not being asked for).