RICH:
The voltage you see on the scan tool is at the
ECU (inside the car), the
ecu is earthed through the chassis (which uses a different earthing path) and is irrelivent to engine sensor earthing (through the block).
The Voltage droop we're talking about is throught the engine block, subarus are renown for poor engine earthing.
A simple piece 4ga (6mm) wire will do to test the earth issue if it exists in you case. Simply run it from under a bolt on the manifold (i.e a vacuum hose support) to the negative (-) battery terminal and see if those codes re-appear.
It's a $0 dollar fix if it resolves your issue.