That sounds like a bug to me.  Or as the gentleman who sold me my used Beetle would say, a feature.
Jeff

> Date: Mon, 24 Jun 2013 17:07:28 -0400
> From: dbeierl@ATTGLOBAL.NET
> Subject: Re: refrigerator behavior
> To: vanagon@GERRY.VANAGON.COM
> 
> At 05:01 PM 6/24/2013, Jeff Palmer wrote:
> >On my 85 I'm pretty sure I've seen the green LED stay lit when I
> >switched from propane to DC (If I recall correctly).
> 
> The thermocouple goes a fraction of a volt negative to the rest of
> the vehicle when the flame is lit.  This gets amplified by the LED panel.
> 
> When you run the fridge on DC, the current draw raises the LED panel
> ground slightly with respect to the fridge itself, simulating the
> effect of the thermocouple.  Now that I've documented it you can
> consider it a feature that lets you see when the fridge is on DC.  Or
> a bug that prevents you from telling whether the flame is still on
> when the fridge is on DC.
> 
> Yrs,
> d