I can't reproduce on mediawiki.org. On marking a page for translation the first time, I got the correct

"tpt-saveok": "The page [[:$1]] has been marked up for translation with $2 {{PLURAL:$2|translation unit|translation units}}.\nThe page can now be <span class=\"plainlinks\">[$3 translated]</span>.",

and no sign of

"tpt-mark-nochanges": "There is no difference between this revision and the last marked revision.",

But I think I understand what you mean.

Change 213238 had a related patch set uploaded (by Nemo bis):
Make tpt-mark-nochanges impervious to manual addition of <!--T...-->

https://gerrit.wikimedia.org/r/213238

Change 213238 merged by jenkins-bot:
Make tpt-mark-nochanges impervious to manual addition of <!--T...-->

https://gerrit.wikimedia.org/r/213238