I've come to share information regarding the problem that you've encountered when entering GSTIN in QuickBooks Online.
There's an ongoing issue with QuickBooks Online right now, wherein GSTIN disappears after entering and saving it. Our engineers are currently working to determine the root cause of this hurdle and apply a fix.
In this case, I'd recommend contacting our Customer Care Team so you'll be notified whenever any updates on this matter are made available.
You can reach them by following these steps:
Click on Help at the top menu bar.
Hit on the Contact Us button.
Enter a brief description of the issue in the What can we help you with? box.
Press on Let's talk.
Select on Geta callback.
Key in your contact details, then tap on Confirm my call.