Thank you for joining the thread, Gaganbhat. 

Let's turn off and on Custom transaction numbers on your settings and then enter manually the correct invoice number in QuickBooks Online. 

Here's how: 

1. Go to Gear icon and select Account and Settings.
2. On Sales, select Sales form content. 
3. Turn off Custom transaction numbers.
4. Select Save and Done.

You can do the same when turning on Custom transaction numbers. Once done, create an invoice and enter manually the correct invoice number. 

For your reference, you can check on this article: How to change the invoice number on sales forms. 

If the same thing happens, let's check if this a browser-related concern. Cache and cookies make the browser speed up the loading process. However, this can also cause issues when piled up. 

You can use a private window and check if you're able to see the invoice number with prefix. If you do, go back to the main browser and clear the cache. This way, the internet files that may be causing the issue will be deleted.

You can also use other browser such as Google Chrome, Mozilla Firefox or Internet Explorer. 

Feel free to comment below if you need more help with QuickBooks Online. Have a great day!