You split and test every piece?
If you aren't splitting right before testing (ideally on a room temp piece) then testing on the middle of the fresh face(s) then you reading is meaningless...other than the real internal MC will be much higher.
This ^ ^ ^.
That's why the baro door is almost always moving, its reacting to the changing conditions (often wind related)
Furnaces are just not a good candidate for MPD for several reasons, especially because most of them have some auto draft controls, for the reasons listed above, and, because furnaces are usually in the basement, so out of sight out of mind, and the chimney is taller, so subject to more severe swings in chimney draft...which also means its easier to get the flue too cold by the time it gets to the top, instant creosote factory.
I dont recall, maybe you said, but what type of chimney do you have (masonry, (lined?) or double wall stainless insulated pipe?) Also, what size and height?