There hiragana drill has 2 major bugs and 1 minor annoyance:
づ says zu is a mistake(!), dzu isn’t accepted. it says that ず is zu (which is OK) but when it shows the “correct” answer it is zu! so it doesn’t accept zu but recognizes zu is correct. Both kana have to allow zu imo or at least dzu if you wanna be nickpicky.
ぢ says ji is a mistake(!), it says that ji is じ(which is OK) but when it shows the correct answer it is ji! so it doesn’t accept ji but recognizes ji is correct. This is a conflict with じ both things can be right at the same time it’s not exclusive to じ.
ん doesn’t accept n, it wants nn which is weird imo. I was able to figure it out but I wouldn’t need to guess it wants 2 n’s