If there are numbers or special characters included in the string, they should be returned as they are.

Yes, you are missing spaces when returning the string. The first values is not the input, it's the value you return and the other is the expected one.

'Nibvqfhpprffngnyypbfgf' should equal 'Nibvq fhpprff ng nyy pbfgf!'
How is this possible, am I missing something?

I have the same problem, could you explain what you meant by "edge case"? I'd love to do it correctly than to hard-code the answer for the cases that fail.

EDIT: NEVERMIND, the moment I clicked send I realized what may be the case.