In the cold and dark instant action, you can't roll your QFE down low enough to match the airfield elevation. Doesn't strike me as a bug per-se, or a Sinai problem, the Viper just needs a bit more low end range.
Hold the switch up for a second rather than just throwing it. If the switch doesn't stay outboard while bit runs, it'll fail.
It holds magnetically, but you need to give it a second.