This is happening while editing a page. Sections is not activated, but the page has a header with an image set, some content with a couple of inline styles and a Simple CSS entry. I deactivated Simple CSS and the source map issue disappeared. So it looks like Simple CSS is the culprit.
Sorry – looks like a false alarm. I reactivated Simple CSS and it came back. Tried deactivating again, but it was still there.
I can add a little more detail. It is happening on editing of more than the one page – even very simple ones with just a shortcode and title. It shows up in Firefox Quantum 68.0.2 (64-bit) but not Chrome Version 76.0.3809.100 (Official Build) (64-bit) console.