He reckoned that the ECU was reading a lean mixture when on gas and was basically adjusting the fuelling with petrol to attempt to cure it (obviously with no effect as its running LPG) meaning that on the first start of the morning its running super rich.
Sounds like he's about spot on. The fuelling adjustments when on LPG DO have an effect on petrol. The LPG system piggybacks onto the petrol system rather than being completely separate. The amount of fuel going into the engine is adjusted by altering the length of time the injectors stay open for. On petrol this is calculated from the various sensors, MAF, TPS, temp, etc. The LPG system needs slightly longer opening times so it intercepts the pulses to the LPG injectors, adds a fiddle factor and fires the LPG injectors for what it has been told is correct. So if at a given revs and load the petrol injectors need to be open for 10mS and the LPG injectors need to be open for 14mS, the LPG system adds 4mS to the pulse. If this is programmed wrong, the petrol system sees that and alters the fuel trims to get it running at the correct mixture. Then when you come to run on petrol, the mixture is out. Running on petrol for a while will let the trims adjust themselves back to correct.
The Thor engine with Bosch fuel management as you have is very picky about the MAF. They do not respond to MAF cleaner and don't like aftermarket ones, you need to fit the genuine (gold plated from the cost) Bosch ones. If the MAF is reporting wrong airflow into the engine, the mixture will be wrong, it will be giving the amount of fuel that it thinks it needs rather than what it actually needs.
Now you have a genuine MAF sensor, run on petrol only for a couple of days and let the fuel trims settle to what they should be and it should run properly. Then try running on LPG and see if it stays that way or goes out again. If it goes out, the internal map in the LPG ECU (the fiddle factor) will need to be altered.