Sorry
Alex I should have told you about this before, that the lamp needed a few moments of a blockage to activate and then flashes, and -as you now know- does not extinguish until the cleaner has been switched off.
Of the top-range Turbopower3, there were two identical cleaners; 2880 and 2881. The latter worked in the same way as your Purepower, requiring manual override of the autosense each time the cleaner was switched on. The bag full lamp worked just like the Purepower too.
The majority of the 2880 machines had a locking override facility, though I found at least one which was identical to the 2881. However, of those with locking override, I cannot remember for the life of me if the bag full lamp was as your Purepower is. I am inclined to think not.