Quick answer: Construct 3 Platform behavior letting the player pass through one-way platforms unintentionally? The platform may not have the Jumpthru tag, or your drop-through code is permanently disabling collision.

Player can stand on a jumpthru platform, but Down+Jump should let them drop. The drop works but they can’t land on it again until restart.

Use Jumpthru Behavior

One-way platforms need the dedicated Jumpthru behavior, not just Solid. Jumpthru blocks falls from above and allows passage from below.

Temporary Drop-Through

On Down + Jump pressed:
  Player: Set ignoring input… (no)
  Jumpthru: Disable
  Wait 0.1 seconds
  Jumpthru: Enable

Disable the behavior for a frame or two, then re-enable. The player falls through; the platform becomes solid again.

Per-Instance Drop

If multiple platforms overlap, disable on the specific platform under the player’s feet, not on all Jumpthrus. Use platform.UID to target.

Vector Y to Drop

Setting Player.VectorY to a small positive value also drops them through — Platform behavior accepts vertical motion through Jumpthrus going down.

Verifying

Player can stand on jumpthrus. Down+Jump drops cleanly. Player can land again immediately after the drop window closes.

“Jumpthrus need the Jumpthru behavior + a timed drop-through pattern.”

Make the drop-through window very short (0.1-0.2s) — longer windows feel unresponsive when the player wants to re-land.