The sim models icing of various kinds. I've experienced carburetor icing when flying through a cloud with the G.50 (it doesn't have carb heat controls) and the engine died on me.
Other people have also reported that their engine was running fine (no fluctuations in manifold pressure and RPM) and still they were losing speed and altitude, which is a clear indication of icing on the actual airframe (wings and what not).
I guess that would be the cause of your problems, so either try to climb above the troublesome layer of air if at all possible, or descend below it.
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