If you upload invoices for a previous period (e.g., April) in a later return (e.g., September) and pay the GST in that later period, you will likely be liable to pay interest. This is because the tax was not paid by the original due date. Section 50(1) of the GST Act generally mandates interest for delayed tax payments, and this is considered a case of delayed payment due to the late filing and payment of the liability.
23 January 2024
https://taxguru.in/goods-and-service-tax/interest-delayed-payment-tax-section-50-gst-act-analysis.html#:~:text=As%20per%20section%2050(1,50%20is%20automatic%20and%20mandatory.