Rendering has changed since the first report: now, an empty line with a “Insert paragraph” slug displays after 2..

Yet, whatever you click on this slug (i.e. whatever you add an empty paragraph starting item 2), Parsoid strips it. This means: even if you don’t press backspace, Parsoid will merge anyway item 3 with empty item 2:

$ echo -e "#item1\n#\n##item3" | php bin/parse.php --wt2wt
#item1
##item3

However, if item 2 is not empty, the current behavior seems expected, so this could be declined.

Actually, this seems to duplicate T54557, which refers to OpenOffice and Word for external consistency.

What do you mean with “more sensibly”? I can still reproduce the visual issue described in initial report.