It’s neither a known issue, nor am I able to replicate it (I just tested it in Firefox with Tampermonkey and Violentmonkey on Windows).
@nazan, can you go through the steps in this guide (especially step g) to gather more information on the problem? I’m especially interested in the script manager you are using and if there are any errors in the console. And did you have this problem for the entire time since WaniKani changed the review pages, or did it start again only recently?