I tested your setup and this is a real bug and your report is spot on. When a sorter has a pile waiting in its input, it tries to split the whole pile with just one moment's worth of power, so almost all of it comes out heavy (or steel on the eddy). Once the heavy side backs up even a little, the pile never clears and light drops to nothing, even with plenty of power. That's why a new one did the same. I'm fixing it so a backlog still splits 50/50, just at the speed your power allows. Until the update, the trick above does help: make sure heavy/steel always has somewhere to drain, or cut the input for a moment so the pile clears. Really appreciate you taking the time to report it.