Your bounces are not wildly in error, but as they get to the smaller bounces the time problem becomes more visible.
Here's some footage shot at 60fps. The stick I'm holding is marked off in 1 foot increments and is six feet long.Click to view attachment
Watch me drop the ball and the brick then go back and count frames:
How long does it take the ball to fall from top to bottom? I count about 36 frames.
How long does it take to fall the first foot? About 15 frames.
Frame thru it yourself to convince yourself that is the case. I had to. I was surprised it took so long to get started. Almost half the time is taken up getting thru the first foot.
The ball also happens to bounce up one foot. How long does it take to fall back down from the peak of that bounce?
That is 1/6th the distance that it fell originally but it doesn't fall in 1/6th the time, it takes a lot longer.
One sixth the time would be about 6 frames, but the ball falls that foot in about 15 frames, the same as it took to fall one foot originally, starting from the top.
Presuming we have animated the first fall correctly, we can use the expectations it has created to time subsequent bounces.
If the ball bounces back up to one foot, it takes as long to fall back down as the ball took to fall one foot originally.
If the ball bounces back up to four feet it takes as long to fall back down as the ball took to fall four feet originally.
And how long does it take the ball to bounce up
to the peak of each bounce?... As long as it takes to fall back down from that peak. They are almost always symmetrical.
Count the ball bouncing up from the ground, peaking and falling down again. The up and down might be different by a frame because the camera isn't quite catching it at exact moments of impact.
What about the brick? I did that just to show it falls almost the same as the ball. Even though it is much heavier, it might be one frame faster. Maybe air resistance slowed the ball down a tiny bit or maybe the camera is just catching them a hair differently.
Should the bounces still make parabolas in the Y channel? Yes, but they are not scaled copies of the first parabola, they are copies of the first parabola with the bottom clipped off.