Hi kevin,
Sorry for all the inconvenience it may has caused on your side.
Based on the detailed description, it seems like there has only one specific file that are not able to edit. In this situation, please check following details for this specific workbook.
1.Check the official reference: SharePoint files open as read-only - SharePoint | Microsoft Docs
2.Make sure the file is not checked out.(You will see a check-out icon on the corner of the workbook in the library list.) For the reference: Check out, check in, or discard changes to files in a library - Office Support (microsoft.com)
3.May I know the owner of the file like if it is uploaded by other colleague in your organization or by yourself? Ask the owner of the file to make sure the file properties didn't enable the protect settings like Always Open Read-Only before uploading it to SharePoint site.
4.Based on my test, I can reproduce your situation if the file is uploaded by other user in organization and also set the protect status. Then I will have this warning when I open it online. Same as yours, right? Also, there will be a signal in the library list.
5.To better confirm this, ask other colleagues to make sure if they will open the file only in Read-only mode same as your situation, which helps us to narrow down the issue to your side or the file side.
Appreciate your effort and patience on this thread. Share us the results so that we can provide more appropriate resolution for you. Check the details and post back the results whenever you are free and I will be here to offer any support you need.
Best Regards,
Mia