It's at the DM
From a scientific point of view, opening to a holding bag, and probably the backpack and the portable hole, is a watertight door.
Thus, when you "seal" the bag, it is not that the air inside escapes. Otherwise, you would have a thud every time you open the bag by breaking the seal and all the fresh air would penetrate inside.
When a living being is in the bag, there is no lack of air, it lacks oxygen. Living things as we know them breathe oxygen and exhale carbon dioxide. Plant life absorbs carbon dioxide and releases oxygen. We are symbiotic that way. But you can not just throw a fern in the bag and think that it would provide you with oxygen. too little, too slow.
So the pipe you're talking about would work like a snorkel; allowing new oxygen to enter and expel carbon dioxide. It would take more science to determine the size of pipe needed to circulate the volume inside the bag.
The real question is this: Can the pipe stay halfway and out? This also belongs to the SM, but this question also indicates that there is no clear answer in the rules.
My personal decision would be that as long as the bag is not moved, the hose is in good condition.
- The items are not completely sucked when they cross the threshold, otherwise the user would be sucked whenever they would catch something. So, things can be halfway, in part.
- You create, for lack of a better term, a wormhole of our plane in the half-plane each time you open the bag. The ends are relatively fixed in space.
- If the opening goes too far, a new wormhole must be created. So, if the bag is still, the wormhole remains until the bag is completely closed. If the bag is moved, the original wormhole closes with partially contained elements sucked or expelled depending on the mass. When the end points are stable (do not move), a new wormhole is created to connect the two sides.
In your case, if someone was trying to move the bag more than a few feet, the pipe would probably be ejected, unless something inside held it. In this case, a check of the DC20's strength would let it be brought back inside (suppose that there is still volume to hold the whole pipe).