SharePoint Online - file modified date updating when user views files in browser

It's normal because they changed the default open to "edit mode" when opening files. Which honestly, I don't like for this exact scenario. With Auto-Save turned on with the client if you have the newer client, you will run into the same issue as well.

I wish they would have made this an option, I thought there was an option to choose which method, and not sure if it can be overwrote been looking for the message center post about this change to see if they included that option or not.