The actual lumber production can be a mess after you received leftovers from your allies after they left.
1 level 4 mill = 0.25 wps
1 level 5 mill = 1.1 wps
1 level 7 mill = 11 wps
The scenario is, I fell asleep during endless waves, when I woke up, my builder was my only unit alive (if you use wings and fly to walls behind the neutral shop "forge master", even the demonic tanks wouldn't attack the builder). So I picked up the level 7 mills on the ground from the Harbor after my allies left and rebuilt in the Bear Cave.
As in the last picture, I had 4 level 7, 2 level 5, 14 level 4 mills, so 44 + 2.2 + 3.5 = 49.7, but it shows 115.7 ?...even when all other buildings were destroyed long time ago.
But if you test in single player, or you don't get any leftovers from your ally, you still have normal lumber production rate.
p.s. I surf the old posts titled with the key word "lumber" but didn't find any similiar problem other than the mysterious lumber disappeared one. Please delete this post if it's an old bug.