Lagging animations with updaters and self.add

Description of bug / unexpected behavior

Animations behave differently depending on the way the mobject was added to the scene. If I use self.play(Create(mobj)) there is no lag, if I use self.add(mobj) the animation lags in certain cases.

Expected behavior

IMHO there should be no difference between the two cases. Also, if using self.add is discouraged for objects that have updaters, we might consider printing a warning.

How to reproduce the issue

See the following code:

from manim import *

class Test(Scene):
    def construct(self):
        tracker = ValueTracker(-3)
        rect = Rectangle().set_x(tracker.get_value())
        label = Text("x")
        rect.add_updater(lambda m: m.set_x(tracker.get_value()))
        label.add_updater(lambda m: m.move_to(rect.get_center()))
        self.add(rect,label)
        self.play(tracker.animate.set_value(4))
        self.wait()
        self.play(FadeIn(rect))
        self.play(tracker.animate.set_value(-4))
        self.wait()

from manim import *

class Test(Scene):
    def construct(self):
        tracker = ValueTracker(-3)
        rect = Rectangle().set_x(tracker.get_value())
        label = Text("x")
        rect.add_updater(lambda m: m.set_x(tracker.get_value()))
        label.add_updater(lambda m: m.move_to(rect.get_center()))
        self.add(rect,label)
        self.play(tracker.animate.set_value(4))
        self.wait()
        #self.play(FadeIn(rect))
        self.add(rect)
        self.play(tracker.animate.set_value(-4))
        self.wait()

Additional media files

https://user-images.githubusercontent.com/52650214/120992430-e36a4300-c782-11eb-80e4-d228dae55858.mp4