I’ll try to go spoiler-free. :)
The (+) operation is a no-wrap form, so if we apply + on Z it remains Z (a normal + would turn Z it into A).
Also, you may want to use a terminal “value inspector” feature and see how the input changes over the encoding steps (use “tab” key to toggle a terminal mode). Take a closer look to that rot13 transformation.
Keep in mind that it’s a bonus content, I never expected every player go for the compound encoded keywords. :) But it looks like you’re doing great!