Can I know cause of previcious I/O error to superblock detected?

Any errors in btrfs device stats indicate it’s degraded and unraids ui doesn’t indicate it. I’d suggest to clear the device stats, run a scrub to repair, check device stats again if there’s any non zero values clear it again and run scrub again to verify. At that point should be all 0 and the array is resynched, unless there’s a hardware issue or unrepairable corruption.