I’ll add a little more mystery to this. The only stopper I’ve ever had bulge was on a pre-sterilized vial and it was bulging when I flipped the cap off and it was still empty.
I also don’t pay attention to whether old vials are sitting upright are laying down with contents in contact with the stoppers and I’ve never had a problem with any of these stoppers. I think the only time I’ve ever had chunks of stopper in a vial was on a 50ml vial of test cyp 250 that I was drawing out of with a 21g and that included a lot of trt draws of 130mg. So there were a lot of punctures in that stopper. It finally had two or three little chunks appear when there was around 5-10 cc’s left in the vial.