When you issue a refund, eBay credits your FVF proportionally but keeps the $0.30/$0.40 per-order fee.
Full refund: eBay credits 100% of FVF but keeps the per-order fee. Partial refund: FVF credited proportionally.
Yes, eBay credits FVF proportionally to the refund amount. However, the $0.30/$0.40 per-order fee is never refunded.