btrfs linear self-recovering after reboot

root@raspberrypi:~# /usr/local/bin/btrfs-scrub.sh
root: Quick Metadata and Data Balance of /srv/dev-disk-by-label-BTRFS_LINEAR (/dev/sdb)
Done, had to relocate 0 out of 3 chunks
Done, had to relocate 2 out of 3 chunks
Done, had to relocate 0 out of 3 chunks
Done, had to relocate 0 out of 3 chunks
root: Starting scrub of /srv/dev-disk-by-label-BTRFS_LINEAR
btrfs scrub start -Bd /srv/dev-disk-by-label-BTRFS_LINEAR
scrub device /dev/sdb (id 1) done
	scrub started at Fri Nov 24 21:12:38 2017 and finished after 15 seconds
	total bytes scrubbed: 409.66MiB with 0 errors
scrub device /dev/sda (id 2) done
	scrub started at Fri Nov 24 21:12:38 2017 and finished after 1 seconds
	total bytes scrubbed: 1008.00KiB with 0 errors
root@raspberrypi:~# dmesg | grep BTRFS
[    7.528064] BTRFS: device label BTRFS_LINEAR devid 2 transid 119 /dev/sda
[    8.004576] BTRFS: device label BTRFS_LINEAR devid 1 transid 166 /dev/sdb
[    8.390168] BTRFS info (device sdb): disk space caching is enabled
[    8.396164] BTRFS error (device sdb): bad tree block start 10867458048 15195979776
[    8.408157] BTRFS info (device sdb): read error corrected: ino 1 off 15195979776 (dev /dev/sda sector 2080)
[    8.408555] BTRFS info (device sdb): read error corrected: ino 1 off 15195983872 (dev /dev/sda sector 2088)
[    8.409721] BTRFS info (device sdb): read error corrected: ino 1 off 15195987968 (dev /dev/sda sector 2096)
[    8.410262] BTRFS info (device sdb): read error corrected: ino 1 off 15195992064 (dev /dev/sda sector 2104)
[    8.411730] BTRFS error (device sdb): bad tree block start 0 14095319040
[    8.429716] BTRFS info (device sdb): read error corrected: ino 1 off 14095319040 (dev /dev/sda sector 4356800)
[    8.430235] BTRFS info (device sdb): read error corrected: ino 1 off 14095323136 (dev /dev/sda sector 4356808)
[    8.431131] BTRFS info (device sdb): read error corrected: ino 1 off 14095327232 (dev /dev/sda sector 4356816)
[    8.432156] BTRFS info (device sdb): read error corrected: ino 1 off 14095331328 (dev /dev/sda sector 4356824)
[    8.433066] BTRFS error (device sdb): bad tree block start 0 14095237120
[    8.445036] BTRFS info (device sdb): read error corrected: ino 1 off 14095237120 (dev /dev/sda sector 4356640)
[    8.445403] BTRFS info (device sdb): read error corrected: ino 1 off 14095241216 (dev /dev/sda sector 4356648)
[    8.446841] BTRFS error (device sdb): bad tree block start 0 14095384576
[    8.450050] BTRFS error (device sdb): bad tree block start 0 14095269888
[    8.453741] BTRFS error (device sdb): bad tree block start 0 14088732672
[    8.463732] BTRFS info (device sdb): bdev /dev/sda errs: wr 530, rd 63, flush 0, corrupt 0, gen 0
[    8.464971] BTRFS error (device sdb): bad tree block start 0 14095302656
[    8.466029] BTRFS error (device sdb): bad tree block start 0 14095253504
[    8.473182] BTRFS error (device sdb): bad tree block start 0 14095302656
[    8.478776] BTRFS error (device sdb): bad tree block start 0 14094778368
[   69.018702] BTRFS error (device sdb): bad tree block start 0 14095335424
[   69.025796] BTRFS info (device sdb): read error corrected: ino 1 off 14095335424 (dev /dev/sda sector 4356832)
[   69.026180] BTRFS info (device sdb): read error corrected: ino 1 off 14095339520 (dev /dev/sda sector 4356840)
[   69.026552] BTRFS info (device sdb): read error corrected: ino 1 off 14095343616 (dev /dev/sda sector 4356848)
[   69.026919] BTRFS info (device sdb): read error corrected: ino 1 off 14095347712 (dev /dev/sda sector 4356856)
[  116.927436] BTRFS info (device sdb): relocating block group 15195963392 flags 18
[  117.045310] BTRFS info (device sdb): relocating block group 14088667136 flags 20
[  117.504636] BTRFS info (device sdb): found 58 extents