This is the result of using the Child Actor Component.
If you parent 2 meshes directly, they will be auto-welded (by default) which, among other things, would combine their mass.
The auto-welding is not taking place here because the base static mesh does not recognise the fuel tank as a valid target for welding - that actor is wrapped in an additional (child actor) component - those 2 actors simply do not know about each other’s mass.
You can weld them manually like so:
The Weld Simulated Bodies tickbox is important here. Do note that, once welded, Set Mass Override in Kg will override the mass of the entire group, not just the fuel tank. Which may or may not be what you’re after. Do check the numbers and see if they add up correctly.
If that’s an issue, you could consider not welding anything at all and simply have the attached child actors tell their parent directly how much they weigh. And the parent can set their own mass and center of mass. May not be ideal, depending on how accurate the simulation is supposed to be.