The petrol ecu may of re learnt on lpg after seeing a weak mixture on that bank and when on petrol its causing a EML to come on due to the fuelling being wrong.

Not uncommon, usually caused by air leaks, dirty injectors etc.. but unless you have a BMW scanner you can't zero the ecu's info to see if it only does it on one fuel.

I'd say your on the right track though with what Neil has mentioned.

Does it run the funny air bypass system across the inlet manifold ? if so are the lpg pipes before or after that or dip tubed ?

The straight 6's with air bypass have caused a few odd faults in the past