Tom, this is a curious discovery. You are probably a more advanced player than I am.
Generally, the Aaa-ooh (Aaaa-uuuh) exercise is a good way to set the embouchure before playing anything. This could keep old habits from creeping in.
I cannot picture what is happening with your lips between the notes. You are changing something about your embouchure at some point.
If you have a visualizer, practice the air release in a staccato manner in front of a mirror. You could discover what is going wrong.
Where is the tension you are referring to in your embouchure?
Where is the upper lip retreating to?
See if this video helps. You may not be following the process as demonstrated. This is often caused by the body unconsciously employing old habits.
