I hesitated before sending this report, because I thought that the problem was probably just that I hadn’t yet upgraded to WordPress 5.0. But now I have upgraded, and the problem persists.
Basically I cannot edit newsletter blocks using the default visual editor. When I click the floating “pencil” icon next to a block, nothing happens. I can add blocks by dragging them from the “Header”, “Content”, and “Footer” sections but then I am stuck with them how they are. The only way that I can edit is switching into HTML mode. The “X” icon to delete a block, which is next to the “pencil” icon, does work.