An area to check config for:
Administration -> Server -> Tasks -> Task processing
What does
Site administration -> Server -> Tasks -> Task logs
when filter set to 'auto' and drop down set to Success show?
Compare \core\task\automated_backup_task with \core\task\backup_cleanup_task
is clean up romping on backups?
backups will use moodledata/temp/backup/
What, if anything, is in that directory or than 9 byte .log files?
Are there hash named directories and inside those a backup.mbz with
all the xml's and related directories?
There has always been a hard coded number of days to wait before removing anything from moodledata/trashdir/
"the draft files are deleted from the files table after 4 days, the trashed files (not present in files table) are deleted after 24 hours. This is intentional."
From https://tracker.moodle.org/browse/MDL-25592
Also remember that moodledata total space usage is affected by other modules/actions/activities ... like assignments and annotation/grading of assignments - this to say with a space crunch other areas might be involved ... like a teacher that decides to make their own backups of courses ... goes to user private files which admin level users cannot see.
In a space crunch ... like running out of space ... one can manual remove any/all files folders from moodledata/trashdir/ without issues.
'SoS', Ken