If you choose to pay on due date with Bancontact, the payment order will only be offered to the bank on the due date itself. Until that time, the transaction is not known by your bank and will not appear in the list of planned payments in your banking app.
The amount you'll need to pay on the due date will not be reserved on your bank account, so for the payment to succeed, it's important that the balance on your account is sufficient on the due date itself.
On the due date, when the money will be collected from your account, the transaction will show up in the transaction overview of your banking app.
If you're not sure if you planned the payment correctly, you can always re-open the paylink or re-scan the QR-code on the invoice, to check the payment status.