An O ring is properly used when in a fitting, whereby the ring is constrained on all of four sides, across flat surfaces so that it expands into a limited free spaces, and therefore is maintained under constant pressure, in all of four directions.
The troublesome O ring is fitted in an application which correctly requires a gasket which can be compressed in only one direction. With nothing stopping radial expansion, the ring expands and flattens to the extent that sealing pressure is lost. This was bad engineering by Subaru.
Consider these aspects and make sure the O ring is being compressed so as to make an airtight seal against smooth clean surfaces. If in doubt, make yourself a gasket from some suitable impervious, compliant material to ensure a proper seal.
Such a gasket might be a good stock line for on of our commercial members.
P.S. When mine was leaking there was no visible evidence in the form of bubbles or frothing.