Keyframe Stretcher doesn't work properly or what should I do

So, I made new Fusion composition (100 frames) and open it on Fusion page. Make some animation with Transform node (from 0 to 15 frames and from 85 to 100 frames). And set Keyframe Stretcher like this: Soirce Start — 0
Source End — 100
Stretch Start — 15
Stretch End — 84
After that I put Fusion composition on timeline.
If I try change duration in smaller direction — I have black frames from about 80% of Composition
If I try change duration in other direction — sometimes I have first frame like without any settings and nodes and from second frame I have ani,ation
What should I do to avoid this? Thank you!
P.S. I have several strange behavior (or bug) more. Need I make new topik about each one?

HP Z820
2xIntel Xeon X5-2687w
64 Gb RAM
1 Gb SSD ROM (about 3500 mbit/sec)
RTX 3060
Win 11
DVR Studio 17.3.2 build 8

Keyframe stretcher is for keyframes not the duration of the media.
Am I misunderstanding?

Keyframe stretcher is for keyframes not the duration of the media.
Am I misunderstanding?

I don't know, manual say use it for change duration. Or not only I misunderstanding it.