The “temporary” effect is not what Dynalist is doing but rather the browser built-in formatting shortcut. We should really disable that…
The biggest problem of underline is that there’s simply no Markdown syntax left for it. If you want you can write a Stylish script that show itatlics or bold as underline, but in Markdown you can only have 2 ways of emphasizing text (correct me if I’m wrong).
Therefore this is blocked on WYSIWYG: Visual editing (WYSIWYG)