Cool solution, I learned something new!

Imprecise issue. Lack of random tests is raised just above; all languages are not affected.

Nice! I think you could even omit "return".

Same issue in Swift 5.0. After getting attempt test there are many 3 digit input.

Fixed

Python tests throws up only 4 digit strings.

Nice!

That's not a kata issue, should be reported at the official github repository instead.

Favorite short solution, without if condition...

Warning in Swift 5.
Missing example in the description.
I don't see much point in named arguments when they are called just a and b.

Fixed

C# as well

Needs tests. After I solved it correctly, I went back and was literally able to change the return type to String and still pass.

Solved it in Swift, passed all tests, but my solution doesn't show up in the list...

Clicked on "My Solutions", and it shows up under this:

Invalid Solutions:

These solutions originally passed when they were submitted, however the kata's test cases have since been updated and these solutions now no longer pass.

Wuh? (I did it twice in a row just to make sure the test cases were not actually being updated at the same time I was submitting)

You're right, I should have specified. I was also snarkier than I should have been, so apologies for that.

While I am using Swift, I generalized my comment after reading some of the others with the same complaint.

A better suggestion would be to edit the description so that it only shows the portion asserting how the input will be formatted to languages where this is true.