This happens to me too!! Very very annoying. It only happens on lessons for me though, not reviews. Win 7 Firefox 61.0.1
Though perhaps mine is slightly different, I’m not sure. For example, I might be doing the lesson for “Hot Dog” and “Hamburger”. It’ll be on the meaning or reading version of one of those words and all of a sudden it’ll switch to a different word and it’ll be the opposite question (was meaning, now wants reading or vice versa). It’s annoying because sometimes you’ll try to enter the kana and it won’t accept it. The screen will shake and it’ll act like you have incorrect characters on the screen (ex: instead of kana just type F then hit enter and see what happens, thats what i mean). usually to get around this I’ll have to copy and paste or do some weird trick to try and get it to work. Really, really annoying bug.
TBH this is the kind of crap that happens when you don’t chain Viet to his desk