Paid bid upgrades are processed a day earlier compared to voucher upgrades. If at T-72 the availability drops then I believe this means that this is caused by voucher upgrades rather than paid upgrades.

For my last AMS flight I saw C8 drop to C5 at T-96, and then to C1 at T-72.

I do believe that they like to keep at least one C seat available for sale, though they may - possibly - do upgrades on the day of departure for that last seat.