Its not a glitch, its the Physics of the game.
I just did some testing on this. It's definitely some sort of bug, or. For one thing, Ike goes into the "I'm so ****ed and can't do anything state" the moment he goes off the edge from this maneuver, as if he had hit Kirby with the QD in the air. That part definitely seems like buggy behavior to me. There's a reason they made the QD stop at the edge of a platform if you're on the ground.
(As an extra note, this gimping seems to happen with any knockback causing attack that doesn't cause flinching, like Mario's Down-B. Seems to be a general flaw in the recovery state of QD itself. Also, Mario's Down-B doesn't affect Ike at all while he is moving. While doing this in slow-mo training, the water simply passed through Ike during charge. it seems to only push him after he stops and is in the recovery state)
Also, the distance you travel past Kirby on solid ground is proportional to how powerful the QD charge was.
-Fully charged QD with Kirby sucking very close to you (while obviously not being close enough to suck you in before you move) will launch you
about the same distance as an obstacle free fully charged QD (a little more than half the length of FD).
-No charge QD with Kirby sucking very close to you will launch you
about the same distance as an obstacle free no charge QD (about a quarter FD)
If the only thing carrying Ike was the momentum of Kirby's inhale (which should be the case, if anything, because the QD momentum should be stopped when he hits Kirby), I should be going the same distance away from Kirby every time.
(Also, if you're interested, a fully charged QD with Kirby sucking a bit less than halfway across FD will send Ike about 3/4 of the way across the stage. So full QD distance =/= fullQD distance + Kirby inhale.)
And yeah, in case it wasn't made clear already, if this does happen to you in a match, you will
not be able to recover from off the edge. You're completely gimped. I know this isn't a huge meta-game changing concern anyways, but I just wanted to make that clear.
...
God **** this was a long post. Now I need to take a nap.