You are a Private seller so did you use Simple Delivery postage?

What was the refund for , you didn't say,  if for item not received / damaged in transit,  under SD rules eBay pay for this refund,  and you get to keep the payout.

If this was a case for item not as described,  I do understand your argument that it was opened after 30 days, this is still possible,  but the buyer cannot escalate,  so a refund cannot ( should not ) be made.

Are you sure the refund money has left your eBay Balance or bank account back up source?  As said,  cases can be opened after 30 days,   but no refunds will follow,   it cannot be escalated.

@luiza_13