In 1.jpg you can see the lavatory tank is full of water and in 2.jpg the bridge output is at ~8kg, however when more water enters it changes to what is displayed in image 3.jpg. No duplicates have used the lavatory so the water just disappears.

I did try rearranging the pipes to see if maybe having the bridge output right next too lavatory input might have been the cause. However as soon as the pipe backed up to 10kg the water in the bridge output would just start disappearing.

I uploaded my save file, just in case it could be useful.