Sorry. I have no theories.

But you have Example Tests source code, so if I was you I would be looking to debug that in some more detail. For example...

 var got = spaghettiCode(plate);
 // you you debug the value of var got here...
 // you should debug the value of var info.longestIDs here...
 // you should debug the result of info.longestIDs.includes(got) here...
 Test.expect(got != '' && info.longestIDs.includes(got))

...to see what that got really is. I know you say you printed a B but maybe you are fooling yourself and it is really 'B\n' or a 'B ', or maybe you arrived ar the correct answer 'B' but forgot to return it from the function?

The bottom line is 43 other people passed this OK that points to the problem being at your end somehow.

Which test fails? What is the message?