No. You level stamina by leveling positions. Your stamina is the sum of all positions. You lvl positions with films sold. Films sold in better rooms increase, correct.
But you also increase films sold in the same room with the same girl if you shoot another movie. Just look at the number of films sold when you collect the movie and then at the number if you send the same girl again. It is higher. Not high enough to compensate for the increase in position lvl cost, but it is higher.
And I do not know, if it is due to stamina, number of filming sessions, time spent in the studio, total films sold, etc. If it were due to number of films, the short filming sessions would become more interessting, also not resetting.