At *normal* conditions 1 cell moat or 1 cell void is too much for any bitter to cross.
Since you yourself and most of your means for transport (except for trains) can easily cross the 1-cell-gap it is sometimes being used as a tricky defense. Assuming you have mods or scripts that allow you to get your hands on the magic means of water spawning ofc. (splosive means suffice just as well).
Anyways, a word has gotten to me that once you have too many bitters in a crowd they shall evolve into the jumper kind. And teleport their way through...
I am posting this more as a joke and not as the real nasty bug.
but here's the 30 sec no-mods 1.1.21 video that shows you how fast they actually evolve themselves (the speed is only x2 in the final part)
And here's the save file as well. For whatever you might need it.