Oct 6 15:05:27 Unraid kernel: BTRFS info (device sdr1): bdev /dev/sdr1 errs: wr 0, rd 0, flush 0, corrupt 15, gen 0
Oct 6 15:05:27 Unraid kernel: BTRFS info (device sdr1): bdev /dev/sds1 errs: wr 0, rd 0, flush 0, corrupt 9, gen 0
Btrfs was detecting data corruption on both devices, you should run memtest.
Also see here for better pool monitoring, if you start getting more corruption errors after reformatting there is still a problem, and raid1 can't protect from that, only from a device failure.