Hi D0lphi,
No, that's not a bug. When you enter a tournament as an Amateur (an unlimited event for example) and subsequently receive a promotion to the next tier you remain competing in the Amateur tier for the duration of that particular tournament. Pro tier players have access to better clubs, so we needed to add the restriction in there so new Pros would not go back to play Amateur tournaments with Pro level clubs.
To finish out any tournaments you entered while still in the Amateur tier simply take your Pro tier clubs out of your bag for that round and you'll be able to play. Any tournaments that you enter after turning Pro will allow you to use your Pro clubs like normal.