Runny poop is not normal in formula fed babies however runny poop is usually not cause for much concern as long as it doesn't last long.
When your formula fed baby is having runny poop then they're likely having diarrhea which could be due to the flu or other illness.
If the diarrhea clears up after a day or two then the baby is likely okay however if the diarrhea in the formula fed baby lasts longer than a few days then a doctors visit should be made because the baby could dehydrate if they have too much diarrhea.
Formula fed babies should have firm stools that are about the consistency of peanut butter and should not be runny all the time.
However runny bowel movements can sometimes be normal in a lot of healthy, thriving babies that are formula fed but it's a good idea to have the doctor check the baby out anyway just to be sure.