A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Ethan Carter,
Based on your description, kindly try these steps:
1> Remove and re-add the permission
Open the SharePoint document library > select the affected file > More Actions (...) > Manage access > More options (...) > Advanced settings
In the pop-up page, choose your account (affected user) > Remove User Permission
Refresh the browser page, open the SharePoint document library > select the file again > More Actions (...) > Manage access > Start sharing > choose your account email address > elect Can edit.
2> Test the document in Office for the web
Open the document library in a private or InPrivate browser window.
Sign in with the your work account.
Select the document.
Choose Open in browser or Open in Word/Excel/PowerPoint for the web.
Make a small test change.
Save the document.
Confirm that the change appears in SharePoint.
3> Test another browser, device
Please try to open these files on an InPrivate or Incognito browser window, another computer.
I hope the information shared helps point you in the right direction.