Maybe the installation was completed without exchanging the spark plugs and remapping the ECU ?
The spark plugs need to be able to run hotter than a usual plug due to the higher burn of LPG, even tho the petrol spark plug has a longer burn duration the LPG is hotter for a short time.
The ECU needs to be checked or altered to compensate for the spark fire or maybe another sensor reporting the similar issue.
this is not my specialist subject but having recently purchased a FFRR with LPG i have found the installer of my prinz system was not such an expert after all.
Have you noticed that you will sometimes get a transmission judder with the transmission light flashing in the dash speedo area ? - this is also due too the LPG system having a lazy moment - nothing to panic about.