Liquid pumps appear to be deleting portions of mass that they pump.
Build a bottle emptier liquid pump and a liquid reservoir. Also have a pit of liquid with more than 200kg of liquid and a pitcher pump. Allow auto-bottle for the liquid (this ensures that you're getting a full 200kg delivered and makes it easier to identify the bug). When the 200kg of liquid is delivered to the liquid pump the first packet that is pumped will delete some of it's mass. You can measure this by checking the pipe overlay during pumping and also check after all of the liquid from the 200kg bottle has been pumped into the reservoir and will notice that of the original 200kg there is only 192kg left (+30g in the corner sorry for the messy test, I did it again with only 2 tiles clearance for the liquid pump and the same result.
I've noticed the same thing happening with conveyor loaders (.4kg of berry sludge of 10kg total was deleted but I didn't get a screenshot of it and it's less consistent to reproduce)
I also did a quick test with a gas pump and went from 200kg in a bottle to 199.9kg between 2 gas reservoirs.