Why is the amount missing when my customer scans the UPI QR code?

Wcart only fills the amount into the UPI QR code for orders priced in Indian rupees, because UPI only handles INR. If a customer shops in another currency, for example after switching currency on your store, the QR opens without an amount and they type it themselves.

To avoid confusion, add a line in your UPI Instructions on Settings → Store Settings → Payment Gateway → Configure (UPI), for example "Pay the rupee amount shown on your order", and click Save Changes. The instructions appear under the QR code on the confirmation page.