Correct negative current measurement but incorrect positive measurement
if I connect the COM lead to the positive side and the INPUT lead to
  the negative side I get a result of around -23.5 to -25 mA


Your battery is flat (incapable of supplying more current) and you are measuring across the battery which IS NOT measuring the current through the two resistors at all.

By the time you measure it in the reverse direction it has got flatter. I bet if you tried it back the first way it would read even less.

Try disconnecting the two resistors from the supply and letting the meter "join-up" the two disconnected points and that's how to measure current - current is the flow of electricity through wires and not some arbitrary re-wire that bears no relation to what you are tring to measure.

Get a new battery too.