Order number field value can not be overwritten with a merge rule.
In the invoice project, the default fields can not be disabled or deleted. If you use a rule with another value then this value will not be taken to account.
You can create another field and use any rules on it.
To do that disable Order Number from being visible on verification.
Open properties of the field > Recognition > Do not recognize > Verification > Uncheck requires verification
Then create a new field but with a different name example: Order_Number.
Fields > LineItems > Create field > Text > Change name > OK
After that move this field in the order of other Line Items:
Data form > Line items > Properties > Format > Move Up/Move Down
Finally, you can add the rule to merge fields and merge it with the Order Number from the Purchase Orders group.