It's not a bug.

It is in fact a minor bug I noticed today. The default text briefly shows up as the editor is loading before switching to the right font (and it looks like this is independent of whatever Preferences you have set).

The editor layout doesn't look normal to me, have you removed the toolbar through special:preferences or a script?

Well, I also removed the '2010 wikitext editor' toolbar, as enabling it won't have that problem. I just prefer the quick load time of the source editor when editing pages.

Edit: In the meantime, is there another script I can add which enables me to use monospace font?

There is a known bug where the font preferences don't work within the 2010 editor when syntax highlighting is not enabled.

"is there another script I can add which enables me to use monospace font?"

Add this to your global.css:

/*  Source editor font  */
#wpTextbox1 {font-family: monospace;}

Sorry, it still hasn't worked for me, as I get the wiki's default text instead.

That's odd.

I've put that rule into my global.css a long time ago, and I actually don't remember why I did it and where I got it from. But I can reproduce your issue if I:

1. Remove that code from my css

2. Disable the editing toolbar in preferences

When I put the css back, the issue is fixed again.

Are you sure this isn't a cache issue on your side?

These were my settings while testing (normally I have the toolbar enabled):

You made the code invalid by adding a comma. Remove it.

^Oops, I'll change it.

At least that bit works now. Thanks! I'll keep experimenting whenever I wish to.