Personally i’ve noticed the font size and shape changes depending on what i’m using. On my laptop at home it always looks different to my PC and my phone. Not sure if this is intentionally or if it’s a browser thing? But i’ve not seen a recent change, maybe you updated your browser recently and the new version allowed a different font that WaniKani uses?
WaniKani will use fonts present on your system. As @ParalyzedHamster says, this will mean that your font experience will be different potentially between devices and can change if you’ve recenetly intalled the IME, for example.
This has been brought up before and this was a helpful response last year: