I am a bit preplexed by this. It looks like the patch attached to the legend is a FancyBboxPatch which uses the set_facecolor method from Patch. The color is rgba-ified using colors.colorConverter.to_rgba(arg, self._alpha) which will ignore the alpha value passed in (the observed issue). However, looking at the blame for both of those functions, the only changes between 1.3.1 and 1.4 are from #3092 which should not have affected this.

853c26a
calls set_alpha() on the patch; once that is set, set_facecolor uses it to override the "a" in "rgba".

In other words, adding an rcParam changed _alpha of None to a default value; and once that is done, you are stuck with it until you explicitly change it! The painful saga of alpha-handling continues...

As a matter of style, the relevant bit of code in legend:

        if framealpha is None:
            self.get_frame().set_alpha(rcParams["legend.framealpha"])
        else:
            self.get_frame().set_alpha(framealpha)

is obfuscated by the use of the get_frame() method; what it gets is the legendPatch that was created in the same method a few lines above.

I've confirmed that the behavior exhibits after version 1.4.0 (1.3.1 and 1.4.0 are okay, >=1.4.1 not okay)

I have a fix for this, just need to add some tests.