This is illuminated constantly - even with the switch off - any suggestions....can't find anything in the documents.

A little more research after a completely clean install of both P3Dv4 and the aircraft

As long as one of the Condition Levers is at or under 11% - the light goes out regardless of the position of the other condition lever.

I am guessing this is a bug.

 

Sounds like this might be correct.  The auto feather has two modes and operates according to power settings.  I don't know if the low light should be going out when the high light is illuminated but the low should be operating when the power is below 64%.