It can happen in any truck when your FOV/head/seat/mirror position isn't within the required parameters to display it correctly, but its rare.
When inside the truck the exterior of your truck isn't drawn, for performance reasons, and so the mirror has to render a scene that is different to what it looks like when outside, so it switches the truck exterior on/off as required, in the mirror when you are looking at it. This is probably an optimisation to prevent constantly calculating the mirrors with extra info when you are not looking at them.
Example: This is cam 1 when moved outside via extended TrackIR movement, showing no truck exterior.
So there may be a slight performance hit by disabling g_baked_vehicle, but I think your system won't even notice...
luckily for you I had the same issue when play testing this 'mirror adjustment mode' created back in August. The issue was caused when adjusting the mirrors far beyond what you can normally do via F4, but easily fixed... now.
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