System auto create separated promotion code renewal invoice

[Binger]

I'm confused about sometimes some renewal invoices create by the system, it auto separated to promotion code and product into two invoices. it does not happen every time, it happens randomly. Or maybe it was caused by some setting but I can't figure it out.

For example, Invoice#1 details should be included : 
Product A $100, 
Promotion code -50% -$50
Total $50

but, the system will create two invoice:
Invoice #1:
Product A  $100
Total $100

Invoice #2:
Promotion code -50% -$50
Total -$50

I think maybe It's because of the System Cron bug because our daily billing generates tasks too much. Does anyone have some solution to fix it?