This appears to be an iOS bug but it was happening to me tonight, so wanted to let you know Channels is impacted.
Some reports are only happens with Guided Access enabled?
I just watched a one-hour recording, twice: Once on battery and once on the charger. Took care to go nowhere near the tablet the entire time, each time. It played all the way through, each time.
iOS 12.1 on a 2017 9.7" iPad
For me the first time it happened I had Guided Access on. So I then turned it off, and it still happened (I think).