BTRFS in RAID1 is showing double the disk space

All my GUI programs started showing 16TB total space and 8TB free space available, while it should be 8TB total and 4TB available. Any ideas what went wrong and how to fix it?

My btrfs RAID 1 configuration started to show twice as much disk space and free space available, as if my configuration changed to RAID0, but it did not.

I have two identical 8TB disks, I recently upgraded to 5.3.0-46-generic #38~18.04.1-Ubuntu

My /etc/fstab entry looks like this, where UUID is blkid's UUID of /dev/sda & /dev/sdb:

UUID=1b962b21-3130-498b-9543-e84c90f12fdf /mountpoint btrfs defaults 0 0

$ sudo btrfs fi df /mountpoint (this seems OK)

Data, RAID1: total=6.43TiB, used=3.58TiB 
Data, single: total=1.00GiB, used=0.00B 
System, RAID1: total=64.00MiB, used=1.12MiB 
Metadata, RAID1: total=9.03GiB, used=5.19GiB 
GlobalReserve, single: total=512.00MiB, used=0.00B

$ sudo btrfs fi usage /mountpoint

Overall:
    Device size:          14.55TiB
    Device allocated:         12.89TiB
    Device unallocated:        1.66TiB
    Device missing:          0.00B
    Used:              7.17TiB
    Free (estimated):          3.69TiB  (min: 3.69TiB)
    Data ratio:               2.00
    Metadata ratio:           2.00
    Global reserve:      512.00MiB  (used: 0.00B)

Data,single: Size:1.00GiB, Used:0.00B
   /dev/sdb    1.00GiB

Data,RAID1: Size:6.43TiB, Used:3.58TiB
   /dev/sda    6.43TiB
   /dev/sdb    6.43TiB

Metadata,RAID1: Size:9.03GiB, Used:5.19GiB
   /dev/sda    9.03GiB
   /dev/sdb    9.03GiB

System,RAID1: Size:64.00MiB, Used:1.12MiB
   /dev/sda   64.00MiB
   /dev/sdb   64.00MiB

Unallocated:
   /dev/sda  852.97GiB
   /dev/sdb  851.97GiB

$ sudo btrfs device stats shows no errors for both devices and I just finished full scrub, did not solve my problem. All files are accessible, they don't seem corrupt.

When I go to Nautilus, select all the files on the volume and right-click properties, Contents: totaling shows correct numbers, same as $ du -h /mountpoint

Update

Output of $ blkid /dev/sda

/dev/sda: LABEL="B1" UUID="1b962b21-3130-498b-9543-e84c90f12fce" UUID_SUB="58408830-2442-416f-8e6a-e55bfb430764" TYPE="btrfs" PTTYPE="dos"

Output of $ blkid /dev/sdb

/dev/sdb: LABEL="B1" UUID="1b962b21-3130-498b-9543-e84c90f12fce" UUID_SUB="65bd227f-d2c2-4b5b-a73b-a540989c7d41" TYPE="btrfs"

Output of /etc/fstab

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/nvme0n1p2 during installation
UUID=d65dd525-0111-4c6e-a865-b9a912e59e51 /               ext4    errors=remount-ro 0       1
# /boot/efi was on /dev/nvme0n1p1 during installation
UUID=D7F2-12D3  /boot/efi       vfat    umask=0077      0       1
/swapfile                                 none            swap    sw              0       0

# mount BTRFS 2 x 8TB RAID1 
UUID=1b962b21-3130-498b-9543-e84c90f12fce /media/user/B1 btrfs defaults 0 0

# mount old test disks
/dev/disk/by-uuid/1b40a350-7573-4f29-aef6-8be2f34cd111 /mnt/1b40a350-7573-4f29-aef6-8be2f34cd111 auto nosuid,nodev,nofail,x-gvfs-show,x-gvfs-name=TEST2 0 0
/dev/disk/by-uuid/56effa28-df91-4b70-b73e-7665856e829e /mnt/56effa28-df91-4b70-b73e-7665856e829e auto nosuid,nodev,nofail,x-gvfs-show,x-gvfs-name=TEST1 0 0

About the UUID, I formated both disks few years ago with this command: $ sudo mkfs.btrfs -m raid1 -d raid1 /dev/sda /dev/sdb

And output of $sudo btrfs fi show is:

Label: 'B1'  uuid: 1b962b21-3130-498b-9543-e84c90f12fce
    Total devices 2 FS bytes used 3.59TiB
    devid    1 size 7.28TiB used 6.44TiB path /dev/sda
    devid    2 size 7.28TiB used 6.45TiB path /dev/sdb

So I don't know why, but they have the same uuid.