I've also experienced this, with MTV. It's been there up until a couple days ago, when it just vanished, and a movie channel is now in its place. I have a TCL Roku TV, and it was on whatever Channel came before 281 (I think it may have been 280), which now is 275 and says "Roku Movies & TV" and it's full, at least today, with a show called "Cardinal", but I know that's not what was showing the last two days. So I don't know if more channels are moving around, or coming and going, as the numbers of the channels are not showing up consecutively, but with various gaps in the numbers.
I see that MTV is still shown as a Roku channel, however, when looking at it within the icons of Roku channels, it says you need a cable station or other provider, which it never required before, although I think the one I was viewing was not the same as their live cable channel... just something else to make this experience so confusing.