When you open office documents(word, excel, powerpoint, etc.) in sharepoint or in onedrive, they do not get opened in isolation.
Sharepoint and onedrive open document files in their own UI. So, it is not considered as a document file by the browser.
While for regular doc files, they are opened in isolated UI of docisolation.prod.fire.glass.
So, this behavior is expected.
This is expected behavior