Ok, talked with the Havok people!
They pointed that Rapier doesn’t seem to be conserving any momentum (which is not physically correct), because on this example that moves only once, the rapier box stays stuck in midair: Babylon.js Playground (babylonjs.com)
While the Havok version keeps momentum: Physics Drag Havok | Babylon.js Playground (babylonjs.com)
But regardless, they showed me a way to make it behave more like Rapier, increasing the linear damping and gravity factor of the body: Physics Drag Havok | Babylon.js Playground (babylonjs.com). It’s also not updating the sphere position directly, it’s using the sphere as an animated body and updating the velocity instead of the position. 