"New counter for each month" has no effect
https://open-tools.net/forum/ordernumbers-for-magento/5244-number-reset.html
It seems like the "New counter for each month" setting is completely ignored and treated like a global counter...
Workaround in most cases is to use "Separate counter for each Format Value", but that only works when the number format only contains [year] and [month] variables.