@Joeni Thanks for bringing this to my attention. I saw your ping the other day but assumed it related to something I had already fixed.
I rechecked the fix I made last week and have just deployed another update to the API that should resolve the issue.
Background
I recently made an update to our Admin system that prevented subjects from having more than one primary reading or meaning. As a consequence the accepted answer field that is used in the API started reporting incorrectly for kanji as previously all accepted answers were primary readings. The fix I deployed last week resolved this issue when you fetch an individual kanji, however when you fetched a collection of subjects I incorrectly cached a variable that meant all subsequent kanji would use the same value for the accepted answer of the first kanji in the collection.
I didn’t realise this was still an issue until you posted here and on reddit. In future it would be super helpful if you reported the bug to hello@wanikani.com as that would then get logged into our issue tracker and we would get to a resolution much faster.
Once again thanks for posting this here and my apologies for the issues with the API.