Whether bashdown should complete or not for balance reasons, this is 100% a visual bug at least as Billy entirely 'freezes' at whatever frame of breakdown he's in. If a Ven trap catches anyone, their feet should be forced onto the ground where the trap is for visual & hitbox reasons aligning with the expectation that a trap shouldn't make you freeze midair.
Further, the trap does not disarm whom it catches, yet Billy cannot fire while trapped like this. This makes Ven traps further punishing for Billy through this bug rather than by design. I wouldn't complain if it cancels bashdown to leave Billy immobilised yet able to fire like any other hero trapped. This way there is still skill in cancelling abilities with traps by design than 1 specific hero bugged into this 'stun' only.