Hi Kaushik,
Could you please check on whether any Billing Block was removed recently from the preceding documents ?
Any workflow approval process have removed the block and hence the creation of the billing document ?
Since you have the variant saved for 2002 to 9999 - there is a possibility that the old documents can get triggered today for billing document creation. Please let me know if otherwise.
Please check and share your results - will be helpful for our future references.
Thanks and regards