Perhaps I have mistakenly used C-m C-m to create offending equations.(Note the dollar signs). If you want your formula in roman font, mark it and then choose "Roman" from the math panel. This equation "works" by accident, the second does not because it contains an underscore.
How did you create the file? If such files can be created with regular LyX commands, please file a bug on http://bugzilla.lyx.org with a description how it is reproducible.
I now realise this changes the font to simple text which I'm guessing doesn't work with math commands.
This is a bit confusing though as it should either work or not allow math commands to display in the preview when the font is simple text.
Gareth
