Thank you, this comment should be added to the description

The function name "longest" implies to return the longest of s1 or s2, reinforcing the descriptions task of returning the longest string of distinct letters.

Just slightly confusing. When someone asks if you would like tea or coffee they don't expect you to say "Yes".

"Return a new sorted string, the longest possible, containing distinct letters - each taken only once - coming from s1 or s2."

Tests are wrong (or description is wrong).

Tests expect distinct letters coming from s1 AND s2