Which begs the question (at least, it begs it of people like me who are interested in that sort of thing): In English, there is such a thing as a "siege engine", which would be something like a catapult or trebuchet. Is there a similar term in Swedish? I'm guessing it probably doesn't use en motor...
I'm not sure exactly what 'a siege engine' is. Wikipedia gives it as a category in Swedish, belägringsvapen. https://sv.wikipedia.org/wiki/Bel%C3%A4gringsvapen, and English wikipedia also seems to treat it as a category rather than one specific thing https://en.wikipedia.org/wiki/Siege_engine
Anyway that kind of thing isn't en motor. Some mill-like things can be called compound nouns that include vandring or verk. A steam engine is en ångmaskin and some other things may be maskiner too. But both combustion engines (förbränningsmotorer) and electric motors (elmotorer) are motorer.
PS this is what English Wikipedia says about motors/engines: "An engine or motor is a machine designed to convert one form of energy into mechanical energy."
Since that's been the suggested answer all along I can't exactly fix it. There might be some more general bug or glitch that causes accepted answers not to be accepted in some specific cases, but then that's a much bigger issue that can only be solved by Duo devs, not by course contributors. If something like this happens again, please take a screenshot and share it with us so we'll have something to show the developers.
This appears to be much like the Case of the Moose and the Elk (same thing in Swedish although they are different things in real life). Motors and engines are not the same thing in real life. Apparently, we're supposed to use "förbränningsmotor" (combustion engine) to distinguish between them. I do hope Duo will accept this when an engine vice a motor is discussed. (the above example was a multiple choice question, so I couldn't write the answer)
Counterpoint to this on the English side, how often does this differentiation actually matter in every-day conversation? Unless you're in certain lines of work, it's often irrelevant unless it's part of a fixed phrase like 'steam engine' or 'combustion engine'. There are also plenty of places in English where 'motor' is used in this same kind of generic sense ('motor vehicles', 'motorized', etc).
Obviously they're not the same thing, but Swedish really uses the same term for both. You'd likely only ever come across förbränningsmotor in technical literature, if even there. I understand the case for accepting it, but I want to avoid accepting things that would make the speaker inadvertently sound really silly if they tried to use it in natural conversation.