No solution as of now.
About the support ticket which is going on, they are asking me to check if the 3rd party payment gateway I am using is redirecting the customer to the correct URL after the payment. But in my case, the answer is pretty clear. My shopify store has a COD option as well, which is enabled by Advanced Cash on Delivery App, which is developed by shopify itself, and orders processed through this also have the same issue. They are even asking me to check any upsell apps, while I am using none.
The frustrating point is that, if they were investigating the issue from this long, they could have asked me to check these things quite a lot of time back, my support ticked itself is going on for more than 20 days now. The issue happens to be going on for more than a month. And they are asking me check these things now!