Forces are not automatically multiplied by time. When they are applied for one frame only, their effect depends on the frame rate without correction. Alternatively, add a velocity instead of a force.

- Of course we always fix all bugs before implementing new features. However, fixing a bug often requires a new feature, as in this case, physics movement interpolation.

But even without this the physics engine is very stable. You just have to keep in mind to maintain a constant frame rate for using physics. When experiencing a frame rate dependency, set fps_max at a low enough value to avoid frame rate variations.