Must be Video specific, just tested with a random Video. Although I did notice that the “Time left” counter in the bottom right “disregards” full hours, e.g. when I have a 3h5m long video and am at the start it says -05:00 when it should say -03:05:00.
I talked to an engineer who worked on this, he said sometimes videos are encrypted, making it unable to play offline, and sometimes Apple’s api does not even throw an error indicating that.
While we did a good job supporting most video sites, some of them are still blocked by iOS internals