It's not a bug, it is bound to be more inaccurate when the simulation is faster.
Every time step, the position of everything in the simulation is calculated. If you have a planet orbiting a star, and the planet takes 8 days to orbit the star, the simulation will be fine if the time step is 1 minute or even 1 hour (if the time step is 1 hour and the orbit is 8 days = 192 hours, the orbit will appear as a circle with 192 edges). But if you speed up the simulation to 1 day, you will get an octogonal orbit, and it will be extremely unstable. If you increase it more than that, the planet will probably be flung into space.