I still do not know if it exists a rule about it, but I have also seen more examples rejected, and I do not know if these can be alternative conjugations. For example, I have used some from the conjugation chart of -enda, and Duolingo shows some different ("ninaenda" instead of "ninakwenda", or "anaenda" instead of "anakwenda"):
Edit: I think yes, here is a rule!
Polysyllabic verb that behaves as a monosyllabic verb, i.e., retains affix ku- in several tenses
Thanks for your response! :)