OK I have just installed a brand new virgin testforum, no addons, no customisation. The issue is completely reproducible there from IE11, but not other browsers.
At least I have pinned it down to it not being due to any addons or customisation.
OK, that makes me think it must be that user's settings in IE11 maybe? It seems very odd though. She tested it here and it is fine so it's still a bit of a mystery.