This is not a bug, this in the intended functionality. The empty character is highlighted to make it easy for the editor to know that they are adding preformatted text. It makes editing much easier because the character is automatically inserted on every line until you delete it and then it doesn't get copied anymore.
See Preformatted text section on https://www.mediawiki.org/wiki/Help:Formatting for more information.

Close as invalid?

To clarify,

the empty characters from one line duplicates into another line. Also, the text cursor starts at the point between the space and the first word.

I think is also intentional (not just that the initial space is automatically inserted). The idea here being that people sometimes use the preformatted text for multi-line examples, where indentation is important. e.g.

Heading
  Foo
  Bar

  Sub-heading
    Foo
    Bar

Bad example maybe, but anyway if you type this in line by line, you'll see how the automatic indentation makes it easier.

Strange (or weird) that it's intended for it applies to only those using the CodeMirror mode but not for ones not using it. Ah well, close it if you wish.