Weird Voltage measurement problem
The voltage drop is in the wires - if you measure at the red nodes, you are measuring before the wires, so you won't see the voltage drops.

At the red nodes, you are measuring the battery voltage directly - the voltage you measure there won't depend on voltage drops elsewhere in the circuit. (for the pedantic, the voltage at the red nodes may vary with load, depending on the characteristics of the DC-DC converter.)

If the DC-DC converter permits remote voltage sensing, you can solve the problem  by making the converter sense the voltage at the load, rather than at its output.  This would require another pair of wires between the load and DC-DC converter.

An easier solution may be to move the DC-DC converter close to the load.