As to the broad question regarding whether inaccuracte reporting is a "violation" of the FCRA, the broad answer is no.
No statute can mandate human accuracy. To that end, the FCRA defines a violation as having occured when a party reports inaccurate information with the knowledge, at the time of its reporting, that it is inaccurate. See FCRA 623(a)(1).
The FCRA provides two dispute processes for asserting an inaccuracy in reported information, and provides a furnisher the opportunity to then correct or delete any inaccuracy. Once they have been advised of an asserted inaccuracy, they must conduct a "reasonable investigation" and either report the results back to the CRA or directly to the consumer. It is usually only after a dispute that one can raise the issue of knowingly inaccurate reporting, or of lack of a reasonable investigation of an asserted inaccuracy.
If the reporings you describe are disputed and the creditor corrects, then there is no "violation" of the FCRA.
I would first dispute and attempt to get any inaccuracy corrected. Issues of whether or not their reporting was knowingly inaccurate or whether their investigation of any dispute was reasonable may be basis for later assertion of a violation, but not at this time. It is premature to assert a "violation" without first providing the furnisher the opportunity to investigate their reporting and update so as to overcome any inaccuracy.
As for the substance of any dispute, the apparent inacuracy is the reporting of paid when you have not paid the debt.
You can dispute the accuracy of a paid status, and get it updated to remove any reference to paid.
The issue of a score drop based on their updated reporting that the account remains delinquent is normal, as FICO considers an updated reporting on a delinquent account as an affirmation that the period of delinquency has extended/continues thru the date of the updated reporting, and thus affects scoring. I do not see an inaccuracy in the fact that they have updated their reporting to show continued delinquency.
Sorry, I paid it in march. So they updated the unpaid status when I paid if. Same day. It makes no sense.
It sounds like they are reporting it properly. Perhaps you can request they delete the account for goodwill. There are examples on how to do so in the rebuilding section. In the future, it would be wise to request a pay for delete before agreeing to pay. In most of FICO models, paying hurts your score. It is supposed to be fixed in FICO 9. Good luck.