So, in testing, it seems the game doesn't realize when the max for the teir has been hit 100% of the time. I've gotten all parts to 30 and hit the next max, and got a lucky combonation of two deflating rings and a blimp curse ring that slowly pushed me towards the next teir, and yet, despite all my numbers being above 60, and the numbers being 63,65,63, i didn't get the next tier. Instead, i popped. confusing in the least, aggravating in the most.
Viewing post in Yuki Pop Repeat~ (3D Update) comments
Honestly I'm not entirely sure how the max-rank-up thing works, but from what I can tell, you can either focus on belly to boost just that way beyond the usual for a bloat run, the other two for an hourglass run, all three equally to just push from tier B to A normally, but after reaching either bloated, hourglass, or A tier elasticity, you can't go any further without popping unless you happen to have any cowbell item in your inventory, which can turn A tier into "Cowloon", which is the biggest tier I've found (albeit it has the drawback of requiring the lactation curse to be applied which typically means imminent popping). I kinda wish there was an S tier or something like that, but I don't think there is.