On occasion I've seen the governor assign population in such a way that resulted in less overall production, but this is the first time I've seen something quite like this. The governor ignores riverside farms in favor of river-nonadjacent farms. That's just less commerce for no benefit whatsoever. Dropping the slider to 70% for some reason causes the citizen automation to work the riverside farms again. Dropping it to 60% makes it swap back to the non-riverside farms.