I have crated a simple ‘rocket’. It consists from a static mesh with a mass of 100kg and exerts a force of 100 * 9.81 Newtons. In this scenario, the missile hovers in place, as expected.
Then, I added a cube with a mass of 1 KG and tied it to the rest of the rocket via a physics constraint. Now, I’ve changed the booster force to be precisely 101 * 9.81 Newtons, but the missile drifts down quite significantly. I’ve played around with the force and found it to be ~120 * 9.81 Newtons to hover.
The reason I believe this is a bug, is that if I then change the mass of the cube to 20KG, it continues to hover, meaning that the gravity force is incorrectly calculated for objects under 20kg of mass.