Yeah, I'm also experiencing this. It's very annoying. It happens in an actual PvP match in addition to Sandbox. You see the text on screen saying the ult is supposed to be cancelable while it's channeling, but it doesn't actually cancel. Changing the cancel options and cancel keybind does not fix it.
Maybe they don't intend for it to be cancelable? I wonder what the CD would be if you cancel it. Would be important since you're invuln during the channel.
If it's not supposed to be cancelable, they should get rid of the text saying that it is cancellable (it's actually written twice on the screen during the channel).