There's a tab character in the Test.hs for this problem.
To be more specific, the system will never allow a solution being submitted because a simple stderr:
STDERR:
/tmp/haskell117027-18-yv6ht6.z39lr3sor/Codewars/G/Persistence/Test.hs:4:17: Warning:
Tab character
This tab character is exist in both client side example test code and server side test code, and the system misinterpreted the meaning of GHC warning and considered it as a error from the player.
The tab character is right after the statement "import Data.Char" and should be removed in order to make the problem working again.