Debian 11 (Bullseye) comes with the mlt framework 6.24.0 and kdenlive 20.12.3. Unfortunately it was already too late (freeze) to build mlt with enabled OpenCV features, this are also required to use the motion tracker features, which are now still missing in the pure stable release.
So I have just uploaded, migrated and backported mlt 6.26.1 along with kdenlive 21.04.3 to bullseye-backports with opencv support. :-)
If you want to install it, just add the backports repository and install the new version with:
apt install kdenlive/bullseye-backports