Good day, @contactus-solarf.
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:
Lastly, I'm adding this link attached to guide you in managing and setting up GST in QBO: Setting up QuickBooks for GST.
Your patience and understanding in this situation are much appreciated.
Know that I'll be right here to assist you further, should you need anything else. Have a nice day!