Not an exact solution to copy and pasting files but maybe related.
We recently discovered that you can no longer copy and paste images with transparency layers in it. If a user edits the image or makes a drawing change that would cause it to then include a transparency (IE editing in Snaggit Editor) the copy and paste will not work. Cropping so it no longer has a transparency layer fixes this issue.
I suspect when it comes to text copy and pasting, it maybe due to special formatting outside what New Outlook can handle. We discovered similar issues in New Teams as well. Therefore we actually think the main issue stems from WebView2.
Example below of transparency layers