An user publish an item.
Later, he unpublish the item by editing it from frontpage editor.
How can he publish it again?

I just tested in v2.2 and it is the same result as you have, I will post this as a feature request.

This is one of several severe issues concerning frontend editing.
This is another: I have started a discussion at community.getk2.org/forum/topics/itemarticle-not-editable-when?commentId=3536014%3AComment%3A59908 on the problem that items is difficult to edit when author alias is used. The item doesn't show on "My Page". But no one seems to take any notice of this thread.

I am having great difficulties implementing K2 in a magazine website with 4-5 frontend editors.