Valley 1:Overflow Vents
Sometimes this happens, and it's unavoidable. We try not to have this happen, but given that this is a procedural game you get a number of combination effects on things.
In many cases, what happens is that there are too many doors to actually fit them all into the first room that had a vent. So it puts a vent instead, since it can fit that, rather than just having no passage at all to the staircase. That doesn't happen super often (usually the vents are actually side passages), but this is kind of an overflow valve for when this sort of thing does happen.