I made some more test with save editing and console.
During gym talk you have 3 options:
Like this - CAIT_GYM - 1 - tooltip suggest you will lock Cait in current state,
All the way - CAIT_GYM - 2 - tooltip suggest she will go back to basic form and stay that way,
Nevermind - CAIT_GYM - 3 - tooltip suggest she will get even more thicker.
After "Like this" - I tested barn duo scene - she still gets more thicknesses resolving in max thick Cait.
After "All the way" - she is still gaining thickness but if you go before she hits 30 she will loose it. After 30 she is locked in Nevermind choice and goes full thick.
I think there are 2 bugs here. First one "All the way" - after Cait gets 30 thickness (basic level) the flag should switch to 1. Second bug CAIT_GYM=1 should block Cait getting more thickness from scenes were she can gain it.
I tested only with barn duo scene.