PayPal Express Integration Issue

I have an issue with InvoicePlane not passing invoice details (description/balance due/ etc) to PayPal. I have set up PayPal Express correctly as instructed but when clicking on a payment link, it shows only the following:

After manually entering the first invoice number, the correct details are now showing up during PayPal Express checkout.

The invoice number only show up in email {{{invoice_number}}}, and the balance due only gets passed to PayPal when the invoice number is manually set.

How can I fix this so that the Invoice Number is automatically applied, included in the email, and passed to PayPal?