In japanese input reviews, if you type something like じつ, then hit left arrow twice to go to the starting position _じつ then type any japanese letter, さ for example, it becomes さじつ_ with the cursor at the END of the string, without you doing it. So if you wanted to correct じつ to さいじつ by appending from the start, you’d instead get “さじつい”. Your cursor got pushed forward after you typed the さ, and you just got a ding on 祭日!!!
This does not happen with Windows Japanese IME, just the wanikani input field.
This was fixed in WanaKana.js (the library used by WaniKani) three years ago. The WaniKani team just needs to update to a more recent version of the library…
That there is a fix is a bit surprising. I’ve thought about this jumping of the cursor issue as well, though I’ve come to just accept it. I usually retype the whole word now for good measure!