Adding @mention deletes the space before @mention

This appears least with the articles:

When you type your text and you add @mention there, save draft or publish it, text will appear like this:

“This is the text and here is the@mention, without space before.”

If I edit the text and I manually add space, @mention jumps to the new row (new row below).

If I edit the text using source code view and I add space there, when I come back to the editor, space has been gone again.

Is there anyone who have seen this?