The only way I can see for that to happen is if for some reason, the pump kept moving water from the turbine to the pipe, but either it is only moving half of it each time, or the turbine is still producing more, even though it has been shut off. I have a good 30 segments of pipe between the nonouple and the big tank, and iirc, only half of the difference in liquid between the two pipes moves from the one that has more to the next one in the line that has less on each tick, so it can take a while for every last bit of it to drain out since the less there is in the pipe, the less the flow rate is. And with the somewhat long pipe, the difference in volume between each segment is small.
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