This is a strange bug. After creating a composite transition with four keyframes for ramp up and ramp down, I now cannot edit these transitions correctly anymore. Trying to move the last two keyframes near and at the end of the transition does correctly move the keyframes in the transition properties frame. However, the underlying MLT model does not correctly get synced, so when playing the transition the dialog shows the edited keyframes, yet the project monitor still shows the old, unchanged keyframes. Moreover, saving and reloading looses all transition keyframe edits; instead, the keyframes get reverted to their original settings.