I am facing the same issue.

I have passed every single individual test case.

But whenever i try to execute two test cases, it shows the error that you mentioned

"returns false when expected true"

There is clearly something wrong with the check

assertEquals(isogram.isIsogram("moOse") , false);

yep, there is mistake

tried doing this in java, kept getting the same error "returns false when expected true" even when i changed every return to return true... something is broken here, but i know my solution should work, it worked for the random tests but not the fixed one so i couldn't pass