Hey and welcome in the community 
!
Unfortunately this is a known problem.
Here you can read more about it: Opening files in microsoft apps still just read only · Issue #150 · cryptomator/android · GitHub
Unfortunately we can’t do anything without reducing the security of our app. Microsoft must patch their applications 
Edit: You can still edit these files by exporting, opening and uploading it after changing to the vault. Not convenient, I know 