btrfs df reports my 100Gb device is almost out of space (which agrees with the results produced by the standard "df"):

root@next:/home/support# btrfs fi df /
Data, single: total=82.00GiB, used=78.23GiB
System, DUP: total=32.00MiB, used=16.00KiB
Metadata, DUP: total=1.00GiB, used=153.70MiB
GlobalReserve, single: total=68.45MiB, used=0.00B
root@next:/home/support# df -h /
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 96G 79G 16G 84% /

root@next:/home/support# du -hd1 /

70M /boot

0 /dev

2

.2G   
          /.snapshots

14M /bin

4

.5M   
          /etc

2

.5M   
          /home

348M /lib

4

.0K   
          /lib64

0 /media

0 /mnt

0 /opt

0 /proc

40K /root

2

.7M   
          /run

12M /sbin

0 /srv

0 /sys

0 /tmp

566M /usr

5

.0G   
          /var

29G /next

38G /

I.e. almost 40Gb just gone somewhere. Am I
          doing something wrong? Is there a problem or a piece of theory
          I'm missing? Kindly advice.

+++++++++++++++++++++++++++++++++++++

root@next:~#  uname -a
Linux next 5.10.0-28-amd64 #1 SMP Debian 5.10.209-2
              (2024-01-31) x86_64 GNU/Linux
root@next:~#  btrfs --version
btrfs-progs v5.10.1
root@next:~#  btrfs fi show
Label: 'NEXT_ROOTFS'  uuid:
              abc71bdb-c570-461d-a28a-54294a646089
        Total devices 1 FS bytes used 78.37GiB
        devid    1 size 95.46GiB used 84.06GiB
              path /dev/sda3
 
root@next:~#  btrfs fi df /
Data,
              single: total=82.00GiB, used=78.22GiB
System,
              DUP: total=32.00MiB, used=16.00KiB
Metadata,
              DUP: total=1.00GiB, used=153.64MiB
GlobalReserve,
              single: total=68.45MiB, used=0.00B