Whatever tier you enter an unlimited play tournament in is the tier you will continue to compete in for the duration of the tournament, regardless of whether you've moved up in the interim. That's not a bug, that's by design. The "bug" is that now there are Master-only clubs that become available to people who have moved up from Pro to Master. They can then use those Master-only clubs to compete in the Pro tier tournament they entered before they became Master.
To put it clearly: Player X starts the week on Monday as Pro, enters the unlimited play weekly tournament, and at some point during the week becomes a Master. Player X can now go buy those PIng G10 Master irons and the R9 8.5 degree driver. Since Player X entered the unlimited-play weekly tournament as Pro, every score recorded for the week will be in the Pro division. Unfortunately for the rest of the Pro tier, Player X has Master level irons and driver now, and can easily better their score.
Note, this "bug" will also affect any monthly
unlimited play tournaments as well. If you entered the monthly tournament on the first day as a Pro and turned Master the same day, you'll still record scores as a Pro for that tournament for the rest of the month.
My prediction is that unless this issue is addressed, every week we will see a Pro-turned-Master winning the Pro division tournaments handily with their newly purchased 8.5 degree driver and Master-only irons in hand, thus defeating the purpose of making certain clubs Master-only to begin with.