After the latest patch on tanning, my tubs have gone crazy. I have three. After my first batch of leather after the patch, all of them remained at full fluid though they all produced 4 leather. After my second batch one remained full, one used about 1/10 and the other 2/10 of the fluid, though all of them produced 4 leather. Can't say how it works, but seems bugged. I used random skins, 1x1, 2x1 and 2x2, and can't tell which one was in which tub.
Edit: They weren't refilled with fluid or lifted since the patch, so maybe they didn't update properly or something?