More of an inconsistency that a full on bug.

Arrows, tridents, and crossbow fireworks shot through a floating tripwire grid (in other words, a tripwire grid with air beneath the tripwires) are detected unreliably, often being entirely missed if the projectile is fired at high power and/or perpendicular to the grid.

It would be expected that entities falling (or being fired) through the tripwire grid would be detected regardless of speed (slow arrows especially are much more likely to be detected).

This can be reproduced by creating a set of tripwires in a grid pattern and firing a projetile through it, if it is fired close to the grid, at a low angle, and/or low speed it is likely to be detected. At a high speeds and high angles it is very likely to sail through undetected 

A more reliable way of seeing the unreliability with arrows is by setting up a dispenser arrow machine gun a decent distance above the test grid, which when fired will cause the grid to flicker on and off randomly.

A world has been provided with these functions already built for ease of testing.