moodle data folder grows after backup

by Stefano Buelow.  

Hi
I'm experiencing a strange behaviour on the data folder.
These are the relevant versions:
Moodle: 4.0.6
PHP: 7.4.33
OS: Red Hat Enterprise Linux release 8.7
DB: MariaDB 10.2.38 (on a remote server)

We have automated backup active, running every night at 00:50.
The backup stores .mbz files on a network folder, so there's only a small footprint of the backup process itself inside the temp dir. A cleanup process recollects some of the space later, around 7:00 in the morning.

The issue here is that every night we "lose" a significant amount of free space for no apparent reason.

The attached screenshot depicts the situation (data extracted with zabbix from server, time span is about 3 days). As you can see every night (the grey area) the backup starts and lasts for about 1 hour. The cleanup job wipes temp data generated during the backup, but the free space remaining is not the same as before.
Also, the graph here is represented in percentage, but the filesystem is specifically dedicated to Moodle data and is 760Gb in size. This means that every night we "lose" about 20Gb of free space.

I've already performed some checks with moosh with file-datacheck and file-dbcheck commands.
File-datacheck returned no errors. File-dbcheck listed abous 60 files that are not tracked in the DB. But the cumulative size of these files does not exceed 700Mb. Surely some space to take back, but not conclusive.

Scheduled tasks seem to run without errors.
Honestly I have no idea what/where to look for anymore.

Does anybody have any suggestions ?
Thanks in advance