Might this point to a power management issue where the unit is not properly powered off when it should be?
No, the power management feature is behaving perfectly. The unit is not fully powered off, but rather in a super power saving state called stop mode.
The reason you see positions is because during stop mode the unit wakes for a moment every minute to evaluate its own state. AVL's, config events, etc. will be logged as normal during these short wake periods. These events will not be sent until comms is active again. These AVL's that you are seeing in the Last Position field in MiX Fleet Manager are only being received long after they occurred when comms became active during power boosts to commission.
We expect to see no updates after, e.g. 80 hours (since 10 x 8h resurface = 80 hours). Why are we seeing updates on positions, etc. long after the estimated 80 hour cut off?
You are seeing these updates, because the power management will boost comms when commissioning is due every 24 hours.