I think, sadly, you have no replies because no one is experiencing this problem. I've been successfully using wifi calling for years and just don't have an answer for you, I'm sorry. Maybe reset your device completely if that's an option.
I have a Note 9 and have the same issue. Sometimes it will last a few weeks before I need to re-setup my wifi calling, other times less than a day. It would be nice if there was a know issue/fix.