You need to produce supporting evidences to the effect that recipient had reversed proportionate ITC against your credit-notes (towards price-reduction along-with GST thereon) as envisioned in proviso to Section 34 (2).
Considering the failure to disclose these credit-notes in From GSTR-1, it would be advisable to have strong supporting evidences specially coming from recipient's end (say, supporting CA certificate at the end of recipient of subject credit-notes & supporting ledger statements).
This is also subject to a presumption that you have adjusted such tax-amount - while filing From GSTR-3B for the month/s - within the time permitted in Section 34 (2).
These are ex facie views of mine, the same should not be construed as professional advice / suggestion.