This query shows all wikis that have changed this message:

https://global-search.toolforge.org/?q=.&regex=1&namespaces=8&title=Discussiontools%5C-signature%5C-prefix

5 wikis had this set just before filing this task, all to add double hyphens. Only one (cswiki) had a space before the hyphens. Two presumably have no space deliberately (zhwiki/jawiki) but two wikis that have spaces by convention did not have it set in the message (itwiki/ukwiki). These have subsequently been fixed.

Adding the space is clearly not intuitive to site admins so should be clearly documented. It can currently lead to T270259 as well.