Low mass components exert greater gravity force

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.

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After some initial testing I’ve found that the issue is with the physics constraint: without it, the problem doesn’t appear (if I just place the cube onto the missile). Moreover, I’ve found that if the missile AND cube have roughly the same mass - then the scenario works, even with the physics constrain and low mass

This is very weird, but swapping the component names solved the issue. So the cube should be component name 1 and the missile body - component name 2. This feels like a bug - why would swapping the components change anything?