Ah that must be because if possible, Makie displays pngs via the text/html MIME type to ensure the correct effective visual size (otherwise with high px_per_unit, images are shown too large). But the notebook frontend only implements the copy button for the png mime type.
You can disable certain MIME types, so you might want to disable text/html if you don’t like this behavior and want the plain png back (which might have sizing issues). Compare to the activation code in CairoMakie, you might need CairoMakie.disable_mime!("svg", "pdf", "text/html") maybe