If I recall properly, the original design for BO-105 fuel system stemmed from a desire to meet Certification Requirements to allow for a Fuel Dump capability to lighten the aircraft following a single engine failure. That required the use of the Supply Tanks to isolate a certain amount of fuel from being jettisoned. The EC-135 is merely a follow-on to the 105 series aircraft.
That is why the Fuel system is the laid out the way it is.
Sometimes that results in "Unintended Consequences".