SUMMARY
When applied on a clip with speed different to 100%, the motion tracker effect will either:
1) Not do anything when clicking "analyze to apply effect"
2) Give a result that makes no sense at all with a simple moving object
3) Perform the tracking seemingly at 100% speed (even if the clip's speed is 50%)
I have never gotten it to work yet. I render the clip at the changed speed and then apply the motion tracking to that as a workaround.

STEPS TO REPRODUCE
1) Add clip with something easy to track to the timeline
2) Significantly change speed (50%, 200%, etc.)
3) Try to apply motion tracking

debug info:

Kdenlive: 25.08.0
Package Type: Flatpak
MLT: 7.33.0
Qt: 6.9.2 (built against 6.9.0 x86_64-little_endian-lp64)
Frameworks: 6.17.0
System: KDE Flatpak runtime
Kernel: linux 6.8.0-71-generic
CPU: x86_64
Windowing System: xcb
GPU: Intel/Mesa Intel(R) UHD Graphics 630 (CFL GT2)
Movit (GPU): disabled
Track Compositing: qtblend