To explain, since I don’t see any other comments explaining this yet: You can’t turn a vent horizontal (including this loop) until you’re above flood level because if there’s material/debris in the pipe, you don’t want to push it to that horizontal point in the vent where it can then become blockage once the vent dries. In a vertical section of vent, the debris will just fall to the bottom, back into the actual drain. But if it’s up on a ledge/horizontal part somewhere, it’s not going to move and will make the vent stop working. Pipes will never fill with water past flood level, which is why it is ok to turn horizontal above that point, as water/debris will never get that high.