Air escaped into negative chamber , so when the build up gets sufficient going through the travel will hit the negative air pressure forcing the dust seal off.
If anything air would have escaped from the negative and into the lowers, its pressure in the lowers that blow the seals off.
Or, on a lighter note, it could just have become dislodged and just need pushing back down.
Doesn’t automatically mean you have an air leak.
Lowers need removing and seal refitting (pushing back down), if its beyond you just send them back.