There is something a bit weird going on for sure. I've seen tickets which have been reissued for whatever reason (schedule change, voluntary change, booked using multiple eCredits, etc) get into a state where DL won't offer even paid upgrades, even though the same upgrades are available on a separate ticket for the exact same flight, same fare class, etc.

I've also noticed that DL blocks many seats in the back of the plane when the seatmap is viewed from a No- or Low-status account, but the entire plane is available to choose from as a PM or higher.

These are all main cabin revenue tickets. Very strange - it's basically the opposite of AS' saver seats being only in the very back.