Drives appear ok, no smart errors, but mirror is degraded?

Hi all, I’m seeing something really unusual. We recently were copying a very large amount of content from a backup server to our FreeNAS box, and after a few hours it failed. Then I got some error notifications and a degraded Raidz array.

I checked the physical HDDs and they were all fine. The drive in question didn’t fail any smart tests and appeared to be working. I used zpool clear to clear the error, which it did and returned the drive back to ONLINE status.

However I’m still seeing this in the Mirror section which is being reported to me each day:

Checking status of gmirror(8) devices:
        Name    Status  Components
mirror/swap0  COMPLETE  mfisyspd7p1 (ACTIVE)
                        mfisyspd6p1 (ACTIVE)
mirror/swap1  COMPLETE  mfisyspd5p1 (ACTIVE)
                        mfisyspd4p1 (ACTIVE)
mirror/swap2  COMPLETE  mfisyspd3p1 (ACTIVE)
                        mfisyspd2p1 (ACTIVE)
mirror/swap3  DEGRADED  mfisyspd0p1 (ACTIVE)

-- End of daily output --

When I run zpool status -v I get this:

root@freenas:~ # zpool status -v
pool: Main
state: ONLINE
scan: resilvered 20K in 0 days 00:00:15 with 0 errors on Sun Apr 3 19:37:54 2022

NAME                                            STATE     READ WRITE CKSUM
Main                                            ONLINE       0     0     0
  raidz2-0                                      ONLINE       0     0     0
    gptid/d71f47be-0224-11ec-888b-ecf4bbc00518  ONLINE       0     0     0
    gptid/d0b3f120-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0
    gptid/d2077e13-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0
    gptid/d31d68f2-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0
    gptid/d42ef454-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0
    gptid/d544f8e2-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0
    gptid/d657a523-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0
    gptid/d7776b20-cfd2-11e7-8d06-ecf4bbc00518  ONLINE       0     0     0

errors: No known data errors

pool: freenas-boot
state: ONLINE
scan: scrub repaired 0 in 0 days 00:01:39 with 0 errors on Mon Apr 4 03:46:39 2022

freenas-boot  ONLINE       0     0     0
  da0p2     ONLINE       0     0     0

errors: No known data errors

Any ideas why this would show only in the case of a mirror, and is this indicative of something I need to do, or is there a way to clear this error in the mirrors?