When a case is opened it is all automated, no human contact so not worth challenging it at that point but to accept and refund before eBay do, which can result in a damaging defect on your account.
If the buyer has tried to repair this item, it is could be seen as being sent back in a different condition than it was, when sent out, thus breaking a rule for the refund in eBay's 30 day MBG policy.
You could use this and appeal the decision for the refund. It's not easy to win an appeal, but worth a shot if you wish to take this further.
This explains to to appeal.
https://www.ebay.co.uk/help/selling/managing-returns-refunds/appeal-outcome-case-seller?id=4369
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