While this is true it does not have to be the case . You can either inject a lubricant in upstream towards the tank or have something going into the engine via vacuum which many cars use. Of course with a turbo car, a one way valve would be required for this .

Do you have more shots of the LPG and what kit/type of LPG injection it is? I am guessing it is a mixer ring type? I understand that LPG has a higher resistance to knock but of course adding lubrication to it may indeed knock it down in addition to having a high optimum AFR ratio by quite a way over petrol.

But IMO alot of it will hinge on the type of LPG system you are using .