Extroot (overlay) on usb/btrfs not working

Hello. I have tried to use /overlay on two usb sticks in btrfs raid1. Tried 17.01.02 and 4902 snapshot on R7800.

According to manual, I set /etc/config/fstab to

config 'mount'
	option enabled '1'
	option uuid 'bfd78cf9-1c40-4818-b2cb-217c53c78b56'
	option target '/overlay'
	option fstype 'btrfs'
	option options 'noacl,noatime,nodiratime,compress=lzo'

but cannot make it work. When i change target to /mnt, it's okay. It's only /overlay, what is refusing to mount.
If I mount it after boot with mount command and dev device (mount not working with uuid), it mounts.
I have found similar questions, but without answer.

Have someone external overlay working on LEDE?

Thank you