eBay's official policy, even before managed payments, was no FVF credits on partial refunds. I've issued multiple refunds over the years (most often for excess postage when people purchase more than one item but check out separately), and was never once credited any FVF, before or after switching to MP.
Now, people could phone eBay and ask for a manual credit, and more often than not it was granted, but again before MP came in, they announced they would no longer do this.
Some people, however, still had success with getting credited after that announcement, and some have actually had success even after being switched to MP, but this is a result of eBay CS not knowing or understanding the policies more than it is anything else (nothing new about that), so very much depends on whether you get a rep who doesn't know what the official policies are.
PayPal used to refund the respective portion of payment processing fees for partial refunds, but as has been mentioned, this policy was officially changed some years ago, and now they (again, officially) provide no fee credits whether the refund is partial or full. It took quite a while for the policy to roll out, I remember my account still received credits for months after the start date of the new policy while others stopped getting the credits immediately (which was a nice grace period for me, I suppose), but eventually that stopped so now I, too, lose out on the fees if I refund a PayPal payment.
There are a few members who still receive credits, I couldn't say how or why, but they are very much in the minority.
Many payment processors have shifted to this model, I think Etsy is the only one I personally use that still issues fee credits for partial refunds, so it is very much becoming "industry standard" (I hate that phrase 🤣).
I'm not sure I see the sense in requiring return for refund to get fee credits - the point of a partial refund is to avoid the higher cost and inconvenience of returns. I guess if it was an expensive item and fairly hefty partial refund, I could see how it might be more advantageous to go the return for refund, but for most sales the lost fee isn't as much as the total loss of the sale (especially if it was a not as described situation, where to enforce the return option, you'll be out postage both ways just to get $x fee credit), hence why partial refunds remain an attractive option.
