This bug is serious because it causes the most recent backup to be immediately deleted, meaning the only backups retained are older and older.
backup_maintenance() uses ksort(SORT_NUMERIC) to order the files before removing 'stale' backups in excess of the delete_amount value.
SORT_NUMERIC is a poor choice when using file names. It gives somewhat unpredictable results, and PHP 7 effectively reserves the sort order compared with PHP 5.
The current workaround is to turn of maintenance by amount/count. However, if the user isn't aware of this bug, they will think they are doing successful backups, only to find when they need a backup that they only have backups prior to the upgrade to v16.