Why the"Campaign Member Status" set to Paid on the Payment form doesn't reflect on the Campaign member status for the members when they get added to the campaign?
If Campaign Member Status at Payment form has a value and a Campaign is associated with a payment, then a campaign member record will be created and assigned the status specified here. If this field is left blank then they will by default be set to Responded.
If the Campaign Member is created without the Status then
1. If the "Campaign Member Paid Status" field on Payment form has value- Paid
2. Switch to Salesforce Classic, Go to Campaign->Click on Advanced Setup and add the Status value.