Comment 2 for bug 1809838

Now that I play-tested this and once again got it explained by WorldSavior I understand the mechanics. It definitely isn't a bug. The target is used to favor the production of corn flour.

Lets say that the blackroot and corn supply is more then sufficient. Blackroot flour target is set to 15 and corn flour to 30 and stock is 0 for both. The mill then alternating produces corn flour and blackroot flour. When it reaches 15 blackroot flour units it stops producing them and only produces corn flour (twice as fast since the blackroot production slot is used for corn). When 30 corn flour units are produced, it stops the production of corn flour and continues with blackroot. Since blackroot is only processed in mills it doesn't harm any other production sites.

If corn flour would drop below 30, the mills would stop producing blackroot flour and only produce corn flour since blackroot is above the set target.

Makes perfectly sense now. It is a little bit counter-intuitive, because there are no other productionsites with this mechanic.