How to control the sequencer Speed Factor

I wanted to try out how the approach for making a dynamic slow motion would needed to be done in the way that you don’t take the rendered video and just show it at a slower framerate but instead have blender actually interpolate the animations in greater detail.

So I made a cube drop with the bouncing interpolation and tried to make the slowdown happen short before it touches the ground for the first time (frame 15) and reach 10% speed one frame before it touches the ground (frame 22).
But what instead happened was that blender started slowing down time at frame 15 and the 10% point wasn’t at frame 22 but somewhere at frame 60 : (

Since that seemed bugged to me did I write a bug report about it but as expected is it just another unintuitive design choice working as intended. (Or a technical limitation, I don’t know)
I was told instead to make my normal animation in one scene and do the speed factor changes in another that references the animation scene.

Made sense so I tried that, sadly with the exact same result.

So I’m wondering now, how can I get the animations to be slowed down to the speed factor I want at the points where I want it?
As in have the slowdown happen from frame 15 to 23 of the original animation and not 15 to 23 of the slowed down version where the original frame 23 is already somewhere at 60+?
Aside of guessing my way through which seems to be the only way as I see it currently : (

I attached the 2nd attempt with the seperate scenes.
SlowMotionIssue.blend (873 KB)