Linking to my own video as a reference:
http://www.youtube.com/watch?v=A4bJLQwKiqM (2:50 is a good example) (same deal at 9:40, vengeance updating for a bit, then it stops updating after some point, only updating by going downwards on melee hits even though stacks keep going up. eventually it doesn't even update anymore once all adds are dead even though the dot still ticks.)
This behavior has been present since, forever. Sometimes they do generate Vengeance, sometimes they don't. I suspect this has to do with killing the blob that applied the first dot tick or smething similar... but idk :/