While your iPad will have presumably been tested, it is very unusual for a report of a burning smell to be dismissed by Apple technicians.
Your iPad should have no discernible “smell” when being used - especially a “burning smell” that might normally suggest that something was not as it should be. Are you certain that the smell is coming from your iPad?
You should return to the Genius Bar at the Apple Store and express you concern. If necessary, politely ask to speak with the Store Manager. Before visiting, it is advisable to make an appointment:
Genius Bar Reservation and Apple Support Options - Apple
If needed, you can also seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044