Terry, I don't have the venting problem, though my older cells do what yours do on discharge.
I think that when the pack is discharged, there is always one cell that had more capacity than the rest, and it takes the longest to discharge to the minimum voltage. Add to that the low discharge rates of my (Novak) tray, and it seems to take ages for the light to go out. It's just a co-incidence that it is always a middle cell, as mine can be anywhere in the packs I have. HTH
