Invoice splitting

Hi guys.

How can I check config how the system splits the invoice?

Our users process invoice per company, all customers in one go.

Our business user reported that a specific invoice should have been split into 2 immediate invoices with consecutive invoice numbers, but apparently the second invoice got assigned an invoice number ex invoice 1, then the second one is invoice 18.

Can something be done to split them into two invoices with consecutive number range?

I suddenly remember that our number ranges are buffered. Can this be one factor to consider?

Thanks for your time.