Multiple invoices getting generated instead of 1

We are facing this issue very often, Users are moving some lines to awaiting billing and then few lines on next day, so in that case, though considering grouping rules, instead of 1 invoice, 2 invoices are getting generated.

If few lines from Sales Order are moved to Awaiting Billing status on certain date and few lines on another date and ran auto invoice on another date. so in that case 2 invoices will be generated or 1? what would be the date of that invoice?