Large pressure vessels sometimes lose some of the liquids (resets to about 200000 mB) on chunkload or relogin. I tested with 3_3_2 pressure vessel with 3 ultimate capacity upgrades (10080000 mB) and it only happens if you relog while filling pressure vessel with certain piping systems. I've tried;
-EnderIO Ender fluid conduits: LOSE LIQUIDS
-Buildcraft Golden Fluid Pipe: LOSE LIQUIDS
-Thermal Expansion Tesseracts: LOSE LIQUIDS
-MFR Plastic Pipes: LOSE LIQUIDS

But some other piping systems work normally.
-Thermal Dynamics Fluiducts: WORKING AS INTENDED
-Extra Utilities Transfer Pipes: WORKING AS INTENDED

How to recreate:
-Make a Pressure Vessel 3_3_2 size with 3 Ultimate Storage Upgrades,
-Place Creative Portable Tank from Thermal Expansion for easy liquids
-Put some liquid in Creative Portable Tank (I've tested with water and Buidlcraft Oil)
-Connect Creative Portable Tank to the Pressure Vessel with some buggy pipe mentioned above
-Wait until Pressure Vessel fills about 500000mB
-Quit to main menu while Pressure vessel while it continues to be filled
-Reconnect
-Look at the Pressure Vessel GUI for amount of liquid left
-See it's just a little more than 200000mB
-Mourn for the loss off 300000mB liquids.

I'm using Mariculture-Deluxe-1.7.10-1.2.4.2a but it also occured on previous versions.
Also tested with different versions of forge from 1315 to 1388
Sorry for bad engrish.