Same issue, just tried it out in a current project and vertical mode + transitions like soothe-cut produce artifacts when the "speedWarp" option is enabled. If switched to horizontal mode the transitions work fine
I have experienced the exact same issue on two different computers. Vertical Timelines with speedwarp does not work. Remove speedwarp OR switch to horizontal and it works fine. When it does not work, it either goes black or shows what looks like interference or static. Can I provide you with more information on my settings or system setups?