SUMMARY
Rotoscoping simply does nothing. Last known working date was 2024-12-25. Opened my old project and found stuff broken.

STEPS TO REPRODUCE
1. for simple testing, add a colour clip, white background, add to timeline
2. apply Rotoscoping (Mask), mode alpha, alpha operation subtract, invert false, feather width 0, feathering passes 1; in the project monitor, select some random portion

OBSERVED RESULT
The selected portion should become transparent.

EXPECTED RESULT
Observe that nothing happens after the region is made.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ultramarine Linux 43
KDE Plasma Version: 6.5.4
KDE Frameworks Version: 6.21.0
Qt Version: 6.10.1

ADDITIONAL INFORMATION
I am not so sure if some changes were applied to the rotoscoping effect during kdenlive updates, so here's what I've found inside the .kdenlive file for reference (though I don't think this will help):

  <filter id="filter15">
   <property name="filter">rotoscoping</property>
   <property name="mlt_service">mask_start</property>
   <property name="kdenlive_id">mask_start-rotoscoping</property>
   <property name="filter.spline"/>
   <property name="filter.mode">alpha</property>
   <property name="filter.alpha_operation">clear</property>
   <property name="filter.invert">0</property>
   <property name="filter.feather">0</property>
   <property name="filter.feather_passes">1</property>
   <property name="kdenlive:collapsed">0</property>
  </filter>

some property tags for filter.spline also contain pretty large json content, just to make things clear.