Hi Sam,
Yes, so the in_advance mode represents the mode for each recurring monthly invoice.
Also, the 2nd invoice would really depend on the BCD that was set at the time of account creation, so effectively if you see the 1st invoice it would be from 7th Sep till 30th Sep. In absence of BCD, the invoice would have been generated on 7th October. Though the payment triggering on the 7th of Oct for the invoice generated on 1st October is a bit odd. What is the payment method that you are using?
Regards
Karan