Pinning behind the air filter in fruiting bags?

So my two bags of pink oysters, which haven't appeared to be doing much on anything, have suddenly pinned right behind the air vent on both bags. The first picture is a bunch of little guys like I expected it to start out as, but the second bag is one massive stem as wide as the cap! For reference, the air vent patch black clicks on these bags are about 1.25" wide.

Did I do something wrong in sealing the top of the bags, or is it common to see this happen right behind the vent? Maybe I put too much straw in the bags? It's hard to see in the pictures, but they seem pretty healthy, however there has been VERY little mycelial growth in these bags so far and it seems premature to start opening them up. Any suggestions?