Dear Kris Oftedahl,
Good day!!
Apologies for the inconvenience caused and please don’t worry, let’s work together on your concern and move towards a resolution path.
I understand your concern and, in my opinion, the behavior you are facing is not an expected behavior because Excel app or Word app or PowerPoint app in the tab of Teams channel tab will show the My Files of your OneDrive and SharePoint Online sites having access.

However, you are having the different UI when accessing the Office apps in the tab of Microsoft Teams. In this situation, I would like to confirm whether you are having the same behavior in the Microsoft Teams browser:
Open Google Chrome browser or any other browser> Access https://portal.office.com > Enter Office 365 account details> Click on the 9 dots at the top left> Click on the Teams> Go to the Teams tab> Go to the Channel> Click on + > Click on Excel> See whether you are prompted similar to the screenshot as the above.
If yes, then the problem is with the Teams application at your end and I would like to suggest you clear the Microsoft Teams app cache and see the outcome: Clear Teams cache - Microsoft Teams | Microsoft Learn
If clearing the Cache doesn’t have any success, we are sorry and request you to uninstall the Microsoft Teams app: Uninstall Microsoft Teams - Microsoft Support
After uninstalling restart your Microsoft Teams app and re-install the Microsoft Teams app to check the outcome: Download Microsoft Teams Desktop and Mobile Apps | Microsoft Teams (Download Teams for work or school)
If still the same issue, then I believe the problem should be further investigated by related development team via collecting Fiddler logs because there is no any related service incident reported at the Microsoft Service Health dashboard.
I do feel regret for not resolving the problem and redirecting you to the related development team but the fix to your concern can be find out through the Fiddler logs and suggest you raise a support ticket from the Office 365 admin center, where the related team will investigate the problem further by collecting the Fiddler logs.
Reference: Get support - Microsoft 365 admin | Microsoft Learn
Note: To create a support ticket, you need to have admin privileges and if you aren’t admin of your Office 365 organization, please contact the admin of creating a new ticket.
As a workaround till the problem is resolved, please follow the below steps:
Go to the location where the file is stored> Right-click on the Excel file> Copy Link> Use People in organization or people with existing access> Go to the Teams Channel> Click on +> Website> Give a tab name> Paste the copied link at URL section> Save.
Appreciate your patience and understanding. Have a great day!!
Best Regards,
Sophia