If I get a shake for vocab for any reason, I refresh the page to get around the bug. It’s not supposed to happen with vocab. The only time you should get a shake is when you input the untaught reading for kanji.
There’s no need to have them shake. There’s one correct answer (most of the time) and if you don’t type it correctly you are wrong. It’s unambiguous, unlike kanji. With kanji, if you type せき instead of いし for 石, you aren’t wrong, but they want the kunyomi because that’s what you were taught in the lesson. Typing せき for 石 as a vocab word is just wrong.
People used to say it was because vocab - unlike that wretch kanji - isn’t an alcoholic. Turns out, though, they’re both alcoholics - vocab just always manages to get its first drink in before starting work so you never notice the shakes.