Doesn't "mersi" work as well? Or is it only used as "thanks" instead of "thank you"?
Yes, it is more informal
My answer, "Mulțumesc că tu m-ai ajutat ieri", was marked as incorrect.
What do you guys think? Is it right/wrong and why?
No Romanian would use the "tu"as it is redundant, just "mulțumesc că m-ai ajutat ieri". But i won't mark your answer as incorrect. You would be understood.
Is it worth reporting, then? Since it's correct but not idiomatic.