Problem with code validation

My solution of “RECTANGULAR BLOCK SPINNER” does not pass through any validator. It passes all the tests without problems.
What could be the problem with the validator?

I just discovered that the reason is the presence of a newline character at the end of the output. Validation passed after replacing the putStrLn on putStr.
It is not clear to me, why the tests did not see an error in this?

That must be a Haskell-specific problem.
Apparently output is not guaranteed to be flushed until you add a newline.
The usual C++ cout << .... << endl; works fine on both tests and validators.