...what the heck is going on here level 51?

Yup, WaniKani makes little changes to their content all the time, and you can see some recent ones here: WaniKani Content Additions: Ongoing from April 29, 2021

On August 26, they moved 又 from level 2 to level 51. They explained the reasoning behind it in this reply to the thread:

We originally put 又 in a low level because of its visual simplicity, but over the years we noticed that beginners tend to use 又 in simple expressions like またね, which are always written in kana. To avoid the confusion, we decided to make an exception for 又 and move it up to a much higher level! This won’t happen often, it just makes sense in this particular case :slight_smile: