Thanks. I found the said TSB and my car falls into the range of affected VINs for oversensitive-ECU.
In fact the car is set to run very rich mixture when idling (>22:1), which I have also noticed on idle fuel consumption. And it lights up CE after the car running on LPG returns to idle (e.g, I stop at the lights after a period of driving). It seems that LPG system can not keep up in enriching the mixture instantly as required by the ECU.
I do not know why RR choose to run so rich , but it does not make sense in terms of economy.
I am scheduled for a ECU remap on Friday afternoon, I'll see whether it resolves the CE.