His issue is more than likely LPG.
The LTFT learns something ridiciulous with the LPG on, and then he is looking at short term trim, and it takes ages for the LTFT to come back down.
I don't know why you care about -15% when you're running LPG, with LPG it's constantly +25% to -25% and still the wrong mix, especially when engine is not in operating temperature, so run it on lambda control and don't worry about it.
This is not LPG's fault because after turning off the LPG, deleting the Lamda corrections, disconnecting the battery, and driving on petrol alone, it's the same.