This is a bug that has been fixed in versions 20.01 and above, if you need help upgrading then click 'here' . Should you not be in a position where you can upgrade currently there is also a workaround. Please complete the following steps:
Select Purchase
Select Supplier Maintenance
Select relevant supplier
Press Crtl + M
Select Approval tab
Select Approval Method field and press F5 and select relevant code
Select address tab and enter a character in one of the fields
Click OK
Go back to the relevant supplier and delete the character in the address field
The approval method code is retained