Hi @Anonymous ,
I have made a test on my side (not in Government Tenant), and don't have the issue that you mentioned. The screenshot as below:

Set the Items property of the Dropdown1 to following:
'OneNote(Business)'.GetNotebooks()
Set the Items property of the Dropdown2 to following:
'OneNote(Business)'.GetSectionsInNotebook(Dropdown1.Selected.Key).value
Please consider take a try with above solution, check if the issue is solved. Also please re-create a new connection to the OneNote connector, then try above solution again, check if the issue fixed.
If the issue still exists, please consider submit an assisted support ticket through the following link:
https://docs.microsoft.com/en-us/power-platform/admin/get-help-support
In addition, some of the features available in the commercial version of PowerApps are not available to PowerApps US Government customers. Please check the limits within PowerApps US Government as below:
https://docs.microsoft.com/en-us/power-platform/admin/powerapps-us-government#powerapps-us-government-feature-limitations
Best regards,