Lucas grab release -> down tilt doesn't work, and nor does ROB grab release- > down tilt. Almost every character in the game can shield both of these. ROB recovers from releasing somebody on the ground at frame 30, which is also the frame almost every character can act after being broken from a grab, and it only takes one frame to put up your shield. Since neither ROB's nor Lucas's down tilt hits on frame 1, these combos do not actually work on the majority of the cast.
On the other hand, you can actually hit Ness, since he doesn't finish recovering until frame 40, but you won't be able to grab again unless it trips, as once you've hit him it's the same as hitting him with a down tilt in any other circumstance... and it's not like ROB's down tilt always combos into a grab.