BuildCraft version: buildcraft-all-7.99.20
Forge version: forge-1.12.2-14.23.5.2796
Link to crash report or log: no crash
Singleplayer or multiplayer: singleplayer
Steps to reproduce:

1. make an heat exchanger / distiller setup which has a surplus on input oil from a tank. Have the output of the exchanger blocked (full tank). Have the oil be pumped using a pipe pulser from the tank into the heat exchanger.
2. Remember the oil amount in the tank.
3. Leave the game or change dimensions (no chunk loader active).
4. Come back and see that some oil is lost from the tank.

Additional information:
I'm not sure if the fluid loss is on the exchanger or in the distiller. The tanks I use are the buildcraft tank and "Get ya tanks here": https://minecraft.curseforge.com/projects/get-ya-tanks-here.
I have not seen any fluid loss with either tanks in other setups.

Images of my setup:

I can also reproduce fluid loss with 7.99.24.4, except in this case, fluid is being transported by Ender I/O conduits.