I haven’t looked this up to check the detail, but I am fairly sure that if there is an interruption in the bleed air supply to the packs, it constitutes an extra pack cycle, which causes wear in the pack components.
(not sure how or why this wear occurs, but must be something to do with spinning up the air cycle machine).
This is why Airbus tell you to select APU bleed on just before turning off the engine masters.
Happy to be corrected.