Don't worry, @jashan-sharma012. I can help fix your QuickBooks mobile app to calculate GST correctly.
To begin, let's ensure that your app and your phone's Operating system are updated.
If you get the same result, clear the app data to reset to its default state. The process differs depending on the device. Here's how:
In the QuickBooks Online app:
In your device Settings:
If the issue persists, I suggest uninstalling and reinstalling the app.
Moreover, you'll want to learn how QuickBooks Online automatically calculates tax. It includes insights how QuickBooks calculates the total tax rate for each sale.
I've also added this link to guide you generate GST reports in QuickBooks Online.
You can count me in if there's anything else you need. I'm constantly around to assist you. Keep safe always.