MariaDB 10.3.27 Memory usage problem
These are dangerously big; lower them to about 1% of RAM.
tmp_table_size                  = 256M
max_heap_table_size             = 256M

Some of the allocated space gradually grows until it hits a limit derived from a setting or combination of settings.
Do you have anything other than MySQL running on that machine?  If not, I don't expect it to fill up the 8GB.