I have an apprentice moderator who’s just tried flagging a topic to see what happens.
What happens is that it got grayed out/hidden, and I can’t work out how to un-gray it 
Does the fact that he accepted his own flag mean it’s irrevocable?
Just to be precise: I’ve read here about disagreeing with flags, but I don’t even see where in the UI that happens.
And I’m an admin…
I believe if you visit the post, click on the … button and then the wrench, and choose Unhide Post.
Also, the moderator whose action hid the post should have an option to “undo flag”, which should restore the post.
(And you might want to suggest they create unlisted test topics to practise moderating. ;))
Gnng, naff translations again, the French translation of “Unhide Post” round trips to “Show”.
In their defence, the forum is still invite only, but yes.