When changes is brought to the ‘Date’ field, the ‘Rate’ and ‘Amount’ values changes automatically, in the item section

Hi

After having input all details on Sales Invoice and either before or after the sales invoice is saved, when changes is brought to the ‘Date’ field, the ‘Rate’ and ‘Amount’ values changes automatically, in the item section. Note that currency used is local currency, no foreign currency and no exchange rate is applied on the sales invoice. Please advise why this is happening so and how to prevent to rate and amount from changing, whenever some values are changed in other fields on the sales invoice.