This is most likely due to the fact that, when you try an "attemp", your code gets tested against random tests. So, it is possible that sometimes the same code passes more tests than others.

The same solution sometimes works and shows as successful but then it fails without modifying the code, what's wrong?

Yes, finding smarter solutions is good.
Unlike me who's been coding for years but takes sentences very literally, lol.

Thanks for the advice, i'm new on this website.
What is considered a spoiler?
I thought about it before posting it but since the solution was provided, i didn't think it could be an issue, i guess i was wrong :p

Check my solution. I wrote my thoughts in it, i didn't know sets existed in python.

Please use a spoiler flag, comments are visible by default to everyone (I put the flag for you this time).