Allow me to share some information about the default screen of QuickBooks Online (QBO).
When you log out to your QBO account due to being inactive for a couple of hours (depending on the setting you choose between 1-3 hours), the system automatically opens the last session you’re working on when logging back.
This way, you’ll still be able to continue and finish your work. That being said, you’ll want to ensure to sign out to your QBO account for the system to view the dashboard as your home screen when logging back. You can check out this link for more details about the timeout duration in QuickBooks Online.
Nonetheless, if you sign out to your QBO account and the first screen to appear is the invoice screen every time you log in. I recommend clearing your browser's cache. This will delete all the cache from your browser and prevent unusual behavior to occur.