I’m afraid I’ve never encountered your specific trouble before. Because it is likely a rare occurrence, I would suggest contacting wanikani directly. They will probably be able to tell you much more accurately why this happened and what the (possible) consequences are.
You could try turning the setting for if you want to receive wanikani emails off and on again and see if anything happens. It supposedly worked for some people who weren’t getting the emails a while ago.
This worked for me today. I never got the email when I went from 34 to 35, but when I leveled up to 36 yesterday, I got the email for level 35. Turned the setting off and back on and got my new email within a couple of minutes.