We installed 2.73 yesterday on one of our sites, and the custom thank you page is now not showing. It is enabled in the event, and it is there, but it doesn't show any more. Since this is immediately following the 2.73 upgrade, I suspect that (no other changes were made to the events).
I have 2.7.3.a J1.5 loaded and after payment there is no message just a blank area of the screen where the forms previously were and the message should be. Let me know if you find a solution.