Hmmm, in that example, and in the full email source you messaged me (thank you!) the moz-do-not-send attribute should not affect the display of the <img> or <a> tags that that attribute appears in. The mozfilter is only looking at values in the class attribute, and I can’t immediately see anywhere else that it might get filtered.
As such I can’t work out why the link-with-alias’s content and href get separated out, unless for some reason the discourse importer is suddenly deciding to use the plain/text part of the Mime encoded email (which does have the text and URL separated). Why it would do that I don’t know.
In your test discourse setup can you try importing/emailing a thunderbird HTML email with both a link-with-alias and, say an embedded image or something else that will mark it out as the HTML part of the email?