See also vaguely related T12505.

I think the issue here is that the project has no hashtag (by default, milestones do not have a hashtag). Elsewhere, in Owners, we fall back to O123: fulll name which we ignore and just parse the monogram. Projects don't have a similar monogram, and allow commas in their names. In this case we should probably just list the raw PHID, like we do in cases where you don't have permission to see a reviewer, since this will at least make stuff work as expected. Users can go give the milestone a hashtag if they want to improve this, or we could eventually refine it (I think the idea of #parent/25 to indicate "25th milestone of #parent" was tossed around originally).

This is probably the fix, if you want to test it and diff it before I do:

diff --git a/src/applications/project/phid/PhabricatorProjectProjectPHIDType.php b/src/applications/project/phid/PhabricatorProjectProjectPHIDType.php
index 7a04103a7c..c3ea3b599b 100644
--- a/src/applications/project/phid/PhabricatorProjectProjectPHIDType.php
+++ b/src/applications/project/phid/PhabricatorProjectProjectPHIDType.php
@@ -47,6 +47,8 @@ final class PhabricatorProjectProjectPHIDType extends PhabricatorPHIDType {
         $handle->setObjectName('#'.$slug);
         $handle->setURI("/tag/{$slug}/");
       } else {
+        // Blah blah blah, see T12659.
+        $handle->setCommandLineObjectName($project->getPHID());
         $handle->setURI("/project/view/{$id}/");
       }

👍 testing it now.