What's filling /dev/mapper/ubuntu--vg-ubuntu--lv if mounted on /, but total size shows only part of the used space?

I have an issue with a virtual machine running Nextcloud and the version is Ubuntu 22.04 LTS. The system continuously indicates that the logical volume /dev/mapper/ubuntu--vg-ubuntu--lv is full (used 100%). To keep the system running, I’ve had to continuously expand the virtual disk size and extend the logical volume (using this guide ). The logical volume currently has 440GB of size, with 35GB free. Nextcloud storage and a backup disk are mounted in /mnt: Nextcloud storage /mnt/nextcloud is an iSCSI disk on a NAS, currently holding 3TB of data. The backup disk (348 GB) is also mounted in /mnt (/mnt/carpetarespaldosnas).

miusuario@nextcloud:~$ df -h
Filesystem                         Size  Used Avail Use% Mounted on
tmpfs                              3.2G  1.7M  3.2G   1% /run
/dev/mapper/ubuntu--vg-ubuntu--lv  440G  387G   35G  92% /
tmpfs                               16G   28K   16G   1% /dev/shm
tmpfs                              5.0M     0  5.0M   0% /run/lock
/dev/sda2                          2.0G  254M  1.6G  14% /boot
/dev/sda1                          1.1G  6.1M  1.1G   1% /boot/efi
/dev/sdb1                           24T  3.1T   20T  14% /mnt/nextcloud
tmpfs                              3.2G  4.0K  3.2G   1% /run/user/1000

miusuario@nextcloud:~$ sudo du -cha --max-depth=1 /mnt 2>/dev/null
1.3G    /mnt/NCBACKUP-OLD
1002M   /mnt/NCBACKUP
4.0K    /mnt/ncdata
3.1T    /mnt/nextcloud
349G    /mnt/carpetarespaldosnas
3.5T    /mnt
3.5T    total

Since the logical volume is mounted on /, I tried various ways to get its total size to locate what was using the space, but it doesn’t show more than 30G used.

miusuario@nextcloud:~$ sudo du -cha --max-depth=1 --exclude=/mnt / 2>/dev/null
0       /lib64
28K     /dev
0       /proc
4.5G    /usr
4.0K    /opt
260M    /boot
4.0K    /media
0       /libx32
0       /sbin
0       /lib32
308K    /home
3.8G    /swap.img
1.8G    /snap
124M    /root
1.7M    /run
240K    /tmp
20G     /var
0       /bin
0       /lib
4.0K    /srv
16K     /lost+found
11M     /etc
0       /sys
30G     /
30G     total

So, I have no idea what is filling up the disk or what is actually using the space.