This is a bug that has riddled both games for as long as I can remember. I haven't been following Lotro's forums much in the last few years, but I do remember that some years back, the devs requested players to send as much data to the bug reports when this bug occurs -- starting from mouse type and all the usual technical details, activity they were doing etc. So it's clearly a mystery to them too. IIRC, nothing conclusive came out of that thread.
Lots of theories (using the colloquial definition of "theory" here) have been submitted during the years; different mouse types, using the wheel when your camera is in a congested space (e.g. inside a building), interacting with whatever, opening/closing whatever things, different character types and so on. AFAIK, no one has been able to reproduce the bug with 100% certainty. If anyone can say for sure that one/some of these trigger situations mentioned in this thread work, I'd be happy to test them when I have more time. For what it's worth, the bug was present in the other game way before it had any sort of bags introduced (like the collectible, gem etc. in DDO).
But one thing I know for a fact: In this game, the wheel eventually starts working again if I keep playing that same character long enough. It may take 5 minutes or 2 hours, but if I'm persistent enough, I end up noticing that the zoom is suddenly working again. That has never happened to me in Lotro. Maybe this game has more possible "trigger points" for the bug, which could also act as "release points"? I dunno.