Autoinvoice Number in the reverse of Sales Orders i.e Invoice number is ascending order but sales or

Hi,

We are observing a strange behavior in our instance. Normally the AR Invoice Number are assigned in the ascending order based on the sales order number in the ascending order.

Example:

Order - 10000 - Inv No -20000

Order - 10001 - Inv No -20001

Order No -10002 - Inv No -20002

But in our case it is happening the other way and we are not able to find the root cause.

Order - 10000 - Inv No -20002

Order - 10001 - Inv No -20001

Order No -10002 - Inv No -20000

Is there a way to find the reason. We are doing some custom updates in eligible lines for certain field but none of them are related to invoice numbering. It is done separately and before autoinvoice is run.