`AnimationGroup` with an introducer doesn't work when the mobject is already in the scene

[sudgy]

Consider the following scene:

class Test(Scene):
    def construct(self):
        a = MathTex("a")
        self.add(a)
        self.play(AnimationGroup(Write(a)))
        self.wait()

When run, instead of writing "a" like I would expect, the Write doesn't happen at all and the screen is blank for the first second. Making it not in an AnimationGroup fixes it, manually setting introducer to False fixes it, and removing self.add(a) fixes it.

[MrDiver]

This seems like a really weird bug with the animations system