I think I found the culprit. It is the WaniKani Quick Info KT script. Looking at the code, it does some overrides with the item info tab. The code of interest is how it chooses to display the primary reading type for kanji.
Some technical details, so feel free to skip to the end.
Line 133: emphReading = cur.emph === "onyomi" ? cur.on : cur.kun;
It checks if the primary reading type is onyomi. If it isn’t onyomi, it defaults to kunyomi.
This explains the error you are seeing as it doesn’t consider the case where the primary reading could be nanori. Since nanori is not onyomi, it uses the kunyomi for the display text.
I’m not sure if the author of the script is actively maintaining it, but I’ll point them to this thread.