Spot on explanation. If it weren't a fantasy setting she'd definitely be a net consumer of oxygen based on her low surface area to volume ratio for photosynthesis and the fact that she's pretty mobile.
I'd say that having a brain would make that even more of an issue, but given the magic...
This guy is so serious about this off-topic opinion that giving it a strike made them find all two of my other posts in non-related threads just to strike back. Good job, I think? XD
So she'll be pleasantly satisfied with her life and an absolute delight to hang out with as long as you aren't trying to insert yourself into her business? Y'all haven't met many spinster cat ladies and it shows. XD
Lots of possibilities here:
It might not have been made like that on purpose. Student accidents, research mishaps, absent-minded craftsmen. Who knows how many ways there are to screw up magic?
Lack of maintenance. Imagine finding an old vacuum cleaner that doesn't do anything but belch out...