I take that back, I'm a noob. But I'm pretty sure I've seen this happen, it seem to just delete some.
They're exactally the same size and same amount of water, the only difference is the one on the right has liquid wall
stoopid there is one pixel wider on the right
found it out: look at the one with the wall, it is 1 pixel more wide than the left one
basicly its like e-wall when it closes. it deletes whatever is inside it
ID:1436871 hard to explain so i did it with a portal system. its still the same ammount of water but the wall dosnt update while paused which is when im guessing you made the all wall version