In VisualEditor, when the Cite dialog is in use ("Basic Form"), the keystroke equivalent Ctrl+Enter is supposed to close the dialog. It does close the dialog, but it also inserts a newline character into the citation at the cursor position. That insertion should not happen.
Verified on Wikipedia today in the latest Firefox on Windows 7 Professional, and on a MediaWiki 1.28 server using Linux Chrome. The bug is present in MediaWiki 1.28.1 and 1.29.0-wmf.11 (rMWfa8cb272dc79).