I did some more research now - you are correct, the simulation does in fact only run perfectly smooth at 50Hz.
The reason is that Unity's default update frequency for physics is 50Hz. (The physics needs to run at a fixed interval in order to be consistent and framerate independent).
I realize now that this stuttering was actually introduced when I was fixing another issue, which was apparently interconnected with this in an unexpected way.
I'll upload a fixed version as soon as possible since I think this is important.
Thank you for bringing this to my attention!