I'm attempting to set up a sapling farm to feed into my bioreactor. However my harvester keeps harvesting my nearest building, despite the fact that the building is located BEHIND the harvester, which is supposed to only harvest blocks in front of it. What gives?
The harvester will only harvest tree trunks starting in front of it. However, as it begins to harvest a particular trunk, it will look around in a range of about 10x10x30 blocks for further wood blocks and leaf blocks to harvest in order to get the whole tree. This is so that it can harvest an entire tree that may be rather large and have expanded outside the range of area in front of the harvester.
This leads to a slightly "cheaty" farm design that I commonly use: have a 3x3 harvester, but say an 11x11 planter, and anytime a tree grows in the middle next to the harvester, the harvester will actually pick up all the trees surrounding it. Likewise, if you have wood or leaf blocks making up your buildings, they will be picked up by the harvester.
It's a good idea to have your MFR tree farm a good distance away from your base or high up in the sky to prevent these kinds of issues.