If I had to spitball, I'd guess that the issue was caused by the interplay between HTML's whitespace collapse algorithm and whatever method WP uses for source e

If I had to spitball, I'd guess that the issue was caused by the interplay between HTML's whitespace collapse algorithm and whatever method WP uses for source editing. That something is converting whitespace characters to U+0020 for collapse and display, but the conversion breaks copy-and-paste. Whitespace issues in HTML can get quite annoying at times.