This also just happened to me. I look at the Help dropdown menu and it still shows my original Artis Product Key instead of the new Professional Product Key - even though it shows "Studio One 18.104.22.168605 Win x64 (Built on Dec 10 2019). I tried manually activating the Pro Product Key, and it say it was already activated. The opening page banner also shows "Studio One Artist".
I started a Problem Ticket. I am in the USA, so their response time is always sluggish.
A real bummer indeed.