Mar 21 17:21:37 Tower kernel: BTRFS warning (device sdn1): chunk 5466226688 missing 1 devices, max tolerance is 0 for writable mount
Unraid is only detecting two pool members, the other one is being detected as new, and pool is mount because it wasn't redundant (or at least part of it).
Mar 21 17:16:54 Tower kernel: BTRFS: device fsid 80736f28-b651-4f12-a638-45f718b8a9df devid 3 transid 856585 /dev/sdk1 scanned by udevd (1804)
Mar 21 17:16:54 Tower kernel: BTRFS: device fsid f49492cf-a1df-488c-a31a-19d03ad6f2b0 devid 1 transid 38 /dev/sdu1 scanned by udevd (1866)
Mar 21 17:16:54 Tower kernel: BTRFS: device fsid 80736f28-b651-4f12-a638-45f718b8a9df devid 2 transid 856585 /dev/sdn1 scanned by udevd (1822)
And the above confirms that sdu is not part of the same pool, fsid is not the same, something happened to this device and it was re-formatted as a different (and newer) filesystem.