I had the same problem with my Galaxy Note 9 and all i did was do a factory reset and hopefully it'll come come like it has on my phone.
Hope this helps
I have a similar problem with my OnePlus 5T since upgrading it to Pie. For me, WiFi calling will only work very occasionally - mostly it defaults back to VoLTE. A work around I've found is to enable airplane mode, then manually turn on WiFi and BlueTooth - i.e. completely disable the mobile stack. I think my problem is specific to OnePlus TBH (and their support is worse than useless), but might be worth a try.
i think samsung changed the wifi calling threshold. i noticed that since i moved to Pie (android 9) on my s9+ that wifi calling isnt active as much. this looks like its because i have a strong 4g signal at home. if you go in to mobile connection settings and set the phone to use 2g/3g (not 4g / lte) then wifi calling should kick in. at least thats what i found. at first i was a bit annoyed but it makes sense.