nymph should be an option for all Mantodea. Without a URL or a screenshot I can’t say whether or not there’s a bug. And as @marina_gorbunova said, there isn’t a pupal stage for mantids.
Then we’d have larva and nymph as options for observations, at class Insect, even though one or the other might not be appropriate for the observation’s subject, correct? I guess that’s what we now do for gall, although there are calls to make it even more open. In my experience it makes sense that we err on the side of gall being more available, even for situations where it’s not appropriate, because there are people specifically looking for galls to identify. I’m less sure that there are people looking for all insect larvae or nymphs and identifying them, but I may be wrong.