This bug happens when you attempt to move liquids in a rocket's interior while simultaneously using its exterior rocket platform's liquid input/output ports.
Steps to repeat:
1. Rocket Platform + Liquid tank + Spacefarer Module/Solo Nosecone
2. Build a Liquid Loader and a Liquid Unloader on the rocket's platform. Add manual automation switches to turn these on/off. Use these to fill the rocket's liquid tank to an amount you choose.
3. Build both a wall mounted liquid input and output inside the rocket. Add automation to the wall-input with a manual switch (set to OFF), then build a short pipe run from its wall input to the wall-outlet.
4. Connect the rocket platform's liquid loader and unloaders together (you'll need power).
5. Watch the rocket's liquid tank level. Turn on the loader and unloaders. They'll loop liquid inside of the rocket's liquid tank.
6. Go inside the rocket, and turn on the wall-mounted pump.
7. Observe the liquid in the pipes slowly disappear into the rocket. The rocket's liquid tank level should now be fluctuation between 0 and 10kg, and any other mass within it has disappeared.