Sometimes, when I press f5, the font changes to this. I have some scripts, but none that should affect font.
Technical explanation (not on the Wanikani team, I just do web stuff sometimes):
Wanikani delivers the font it uses (Noto Sans JP) as a web font. If you have internet problems, that can fail to download and then it will use your system’s default kanji font. It’s easier to trigger this when reloading as it causes your browser to cancel any network requests that are in flight.
I have the exact same thing and have mostly been ignoring it. I always assumed it was one of my many userscripts doing something. I quickly checked if disabling some scripts changes anything, and it looks like the community mnemonics script is somehow causing it for me. I assume there could be other, probably older, userscripts that could cause similar issues.
You could easily isolate this on your end by disabling Tampermonkey/half of your active userscripts and seeing if this changes anything.