The first thing I would check is the rich text editor for the label:
There's a little pencil icon to the right of the Label in the form editor which brings up a rich text editor. From there you can drill into the HTML to see if there's strong tags there for some reason. Any luck with that?
The only thing I can think of is that this is somehow related to the form's theme - what if you try a different one? I double-checked with a test form that Marketo does not insert strong tags for required fields - at least not for the theme that I chose.