Have seen this happen quite often to be honest, mainly at Firestorm Citadel Depth 27, Smoldering Steps, when the Wheel Launchers hit Slag Walkers at the bottom of the stage, sending them airborne outside accessible terrain for players.
Another bug involving Slag Walkers (this one is way more rare though) also occurs in Firestorm Citadel, this time in Depth 24, Blackstone Bridge, at the very beginning, when Wheel Launchers may put the Slag Walkers behind the automatic, party button triggered, gate.
As Wheel Launchers are involved in both cases, they seem to be the guilty ones.
And they are kind of annoying ^^
Rather comical bug! But yeah, this probably needs to be fixed - with the profusion of wheel launchers in the citadel a considerable number of crowns could be lost through these bugs