That's because you never hear about the cameras that go offline when nothing interesting happened. It's probably very common.
> normally security would get pinged with alarms especially if the cameras were tagged as critical
Unless you've worked on prison infrastructure before I'm not sure how you could know this with any level of confidence.
Honestly an IT team would MAYBE bother to prioritize these cameras as critical. They only definitely would after something like this happened. It's an easy thing to put on the backburner...