This is an old and strange bug, which is not that harmful, but rather annoying.
One can use the following python code to illustrate it:
from os import system
system('fish')
In fish type exit and see the message:
Job 1, “ipython” has stopped
Then you need to call 'fg' to put the program back to foreground.
The same happens if I use subprocess module of if I use another languages (lua: 'os.execute "fish"').