I know we had a few (extremely few, out of almost 100k) devices with failing PoE supplies (not injector - the actual PSU in the router). I believe ZyXEL identified the problem as a batch of one specific possibly bad component. They have a list of affected serial numbers. It's not as simple as anything lower than X. But they'll know.
You should contact them in any case and request an RMA if this is a hardware failure. That's not supposed to happen.