WIFI calling only kicks in when the phone decides that the signal is not strong enough to maintain voice calls.
it's possible that the voice signal is sufficient to maintain that - or it thinks it is.
my wifi calling on my pixel 5 (on Three) kicks in sometimes in my house, but i've seen the 4G signal extremely low and it still not kick in - mainly because voice won't be using 4G (the H+ signal is reasonably strong).