I am also a Galaxy S9 user and this may just be a bug that Samsung introduced in their update to One UI. Which is the one I think you are referring to. I have been able to fix the issue by putting the phone into airplane mode and then connecting to wifi so it has no options. Sorry if that's not a permanent fix but when it does occur it what I do.
Unfortunately, this is the first post that I've seen around the Support Community regarding WiFi calling taking a back seat to cellular after updating to Pie. We're still working to get answers on why this is happening and what we can do to fix it. In the meantime, we're using this thread: Official Android 9.0 Pie Discussion Thread to track user-reported issues related to Pie. When we have more info, we'll be sure to post it over on the linked thread.