have had the plugin installed for years on more than 40 sites and since December the backup is done on the remote server but also locally even though it is not configured? before December no problem
Can you check if the “Delete local backup” option has been disabled.
Please go to UpdraftPlus ‘Settings’ tab > ‘Show expert settings’ > select ‘Delete local backup’ and make sure to click the ‘Save Changes’ after. This will then tell UpdraftPlus to delete the backup after it is sent to your remote storage