I'm using this seed: 960570313, and the chunk 21 18 keep resetting (entrance of the woodland mansion next to spawn) and this only happens with forge (tried with 25.0.149 and 25.0.215) and no mods. I tried on the same seed in another world, and the same chunk keep resetting. However, it doesn't happen on a normal minecraft instance (no forge).
I tried deleting the chunk in NBTExplorer, and it seemed to work at first, cause first time I logged out and came back, the changes were there. However, if I made another change, logged out and came back, the second change wasn't there. So it seems the chunk can save one time, but not another.