While this bug is annoying, it is also very entertaining.  It is hard to describe so I have included a link to a save file on DropBox and a set of instructions that you can use to recreate the issue:

https://www.dropbox.com/s/ghjp0itmq14tsgn/Subspace%20Fun%20Under%201.51.375185.1.GC4Sav?dl=0

After loading the save file:

1. Go to the area near Subspace Gate 5 (it is located at the top of the lower-left galaxy and should look like the first screen snapshot below).
2. Click on the fleet of 6 ships you should see near there named "The Federation of Free Fish Heze Fleet 1" to select it (it is the only fleet of 6 ships near the Subspace Gate).
3. While the fleet is selected, scroll up along the Subspace Stream until you get to the area around Subspace Gate 6 at the other end of the Subspace Stream (it should look like the second screen snapshot below).
4. Right-click on any cell near Subspace Gate 6 to make the fleet move to that cell.
5. Watch the fleet lose it's mind and travel back and forth across the stream deviating out over the black hole and away from the stream until it eventually runs out of moves.

I am reasonably certain that this is not the intended behavior and I hope this save file helps you find the problem and fix it.