Yes, exactly the same boat and i've reached the same conclusion - i.e. some threshold seems to have changed so WiFi calling keeps dissapearing unless I turn on Airplane mode and then enable Wifi to stop it dropping back to something else. Basically, it only seems to work when connected to a really strong wifi signal if it has a choice of something else. I've tried using Wifi Selector to force a connection always to the strongest AP, which helps as I have multiple APs and otherwise android may well stay connected to a weaker one. Did you make any headway with this?
i have a samsung galaxy s9+. i was on android oreo and wifi calling worked fine. i.e. when i was home on my wifi, wifi calling would be active and stay active till i left.
I have since upgraded to android pie on my s9+ and wifi calling works for a while then drops back to VoLTE. I i disable 4g (and force the phone to 3g only) wifi calling stay active all the time. I'm guessing some sort of wificalling / VoLTE threshold has changed. is anyone in the same boat?