I have a theory as to the root cause...

Somehow the O-ring shrinks a bit (? Ethanol, ? Other chemical, ? Normal aging of rubber BUNA etc.)...then the pressure from the return fuel (probably around 30 psi in the return line) pushes the O-ring out of its groove ---> reducing the flow necessary for the Venturi effect to work.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT