You're not sure a pet chewed it, and eBay won't be sure either, but what they will see if a case for item not as described is opened. For them not to force a refund you need to refund the buyer through the case once opened within 3 days, or pay for a pre paid label for the item to be sent back once returned you have 48 hours to refund your buyer.
Stick within those timelines, and your seller performance is not affected.
If you think it's worthwhile later to challenge the refund, I admit it's not easy as eBay do not see the item involved, then this explains how to appeal that decision for the refund.
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
@blue-dragon88