Some abilities are like this where if you click to use it while out of range, you'll just keep walking up to the thing and not use it on.
Often it's best to babysit anything you said to "throw" an at-nade or at-satchel if you were not in range when you clicked it.
OP's specifically referring to the AT nade assault from tank-hunter conscript upgrade from the doctrine. If the vehicle leaves the range before the animation finishes, then the ability cancels.
It's the only infantry snare to work like that, but that's only because its the strongest. And the squad has camo which can be used to overcome that issue