Just confirming your observations. IE on Windows Phone 8.x has supported both pointer events and standard touch events for some time now and both the
touchend event and the pointerup event are not working correctly. Either they don't fire at all, or they fire irrelevantly while the user's finger is still touching the screen. When the latter happens, it is the same as if they hadn't
fired, because the touchmove and pointermove events are still actively firing.
If you have a routine that is being started by touchmove/pointermove and ended by
touchend/pointerup it will never terminate correctly, because it will be restarted again as soon as the
touchend/pointerup events fire irrelevantly before the finger has left the screen.
The only way around this that I have found so far is to use a timer instead of the end/up events when my web app detects that it is on Windows Phone. It's messy, but there it is.
This is probably not getting noticed because nobody really cares about Windows Phone any more, including Microsoft. I sometimes wonder why I even bother to test my pages on it if Microsoft cares about it even less than the developers trying to work around
their bugs. Sigh...