run_time=tracker.time_until_bookmark("A") not working correctly with custom manim animations?

[Toughbook-CFC2]

class breathe(Animation):
    def __init__(self, mobject, cycles=1, size_factor=1.3, **kwargs):
        self.cycles = max(1, int(cycles))
        self.sizefactor = max(1, size_factor)
        self.OGscale = 1
        super().__init__(mobject, **kwargs)

    def interpolate_mobject(self, alpha):

#        if alpha < 4/10:
#            scale_factor = (1 + (self.sizefactor - 1) * alpha * 10/4)
#        elif alpha >= 4/10: 
#            scale_factor = (self.sizefactor - (self.sizefactor - 1) * (alpha - 4/10) * 10/6)

        self.mobject.shift(LEFT * alpha)

#        self.mobject.scale(scale_factor / self.OGscale)
#        self.OGscale = scale_factor

I wrote a custom animation that would scale a mobject up and then down again, to give it
some kind of breathing motion. The custom animation doesn't seem to stick to the timing of the
bookmark I put into the Text.
Even if I strip down the interpolation to one simple line like this, it
doesn't work and it always overshoots the point of the bookmark in the text.

If I use one of the standard
manim animations like Write it works as intended, an when I put a number into run_time like 2 for
example, my animation works as well. But when I put the voiceover timing together with the custom
animation it breaks.
I think that there is some issue that need handling for building custom animations
to make them compatible with manim voiceover.