No you're not alone, Android Pie (9) seems to be causing problems on a number of brands' handsets.
I also have a UK-unlocked (SIM free) S9+ on BT mobile, and I'm having the same problem since updating to Pie. Here's what I've found so far in 2 different scenarios (both assuming I'm currently connected to wifi):
1) Starting with both VoLTE and wifi calling switched on. I turn Airplane mode on, and wait for everything to disable. I then enable wifi, and the wifi calling icon appears, and I'm able to use it. I then switch Airplane mode off, wifi calling disappears, and VoLTE reappears.
2) Starting with both VoLTE and wifi calling switched off. I enable wifi calling, icon appears fine. I then re-enable VoLTE, all is still fine, nothing happens. I then switch wifi off (or go out of range of my wifi), VoLTE comes back - still as expected. But - now when I switch wifi back on (or go back within range), VoLTE stays, no wifi calling again.
So, the issue here seems to be that once VoLTE is active, it will not give way to wifi calling, as it did before on Android Oreo (8). Some owners of other brands have said they have a 'preference' setting as to which takes prioity, but I can't find any such setting on the S9+. Perhaps this is the issue, and the firmware developers didn't spot it? Who knows.
We just have to hope for an update soon that fixes this behaviour I guess.
UNLESS - The change is intentional in Android Pie, in which case the only explanation I could think of is that it now monitors the quality of the VoLTE and VoWifi connections and selects the best one.
What made me think this, is that I went to a part of the house where cellular signal isn't so good, and I did briefly get the Wifi calling icon back, and when I moved again the VoLTE icon came back.
I have exactly the same problems on Oneplus 5T since shifting to Pie. I think it's a change around some quality bar on Wifi. I see all the same behaviours with Airplane mode, but also if I'm right on top of the access point, then VoWifi will stay connected. It seems to be that Pie wants the Wifi to be > 50db or so, or it won't try and use VoWifi. Anyone else discovered any way around any of this?