Hey again so I'll just note down things as I see them.
Firstly I want to say you have a suuuper nice muscle model there!! I like how all the striations in the muscles are still there, yet the model isn't overly high res. The general shapes also are just really nice. Very cool. Who modeled it?
In the fascia file you sent me, collisions were not turned on globally (on the zSolver node)
I've started with attachments only at the border edges (so deleted all the other sliding and fixed attachments) and I've also halved the object to iterate faster.
One thing that wasn't mentioned was ramping on the pressure and rest scale over 5 frames or so, so the jump isn't too severe on solver start frame. (This is done on the zCloth node, by keying the rest scale and pressure envelope values from 0 to 1 over 5 frames or so)
I changed tensile stiffness from 5 x 105 to 1 x 105. This will stop the fascia from buckling when it's getting compressed (shoulders for instance)
I added a material layer for the armpit and shoulder because the fascia was getting loose there and pushing through itself. In this material I reduced pressure by half (now 20) and set rest scale to 0.8 (was 0.92)
The simulation is breaking around frame 1038 because you have self intersecting muscles underneath. This causes collisions to become unpredictable.