If it's running on petrol, the fuel side of the LPG system is effectively switched off - the injectors remain shut, the valves from the tank remain shut and so on. So yes, the problem is almost certainly nothing to do with the LPG system.
There are of course very rare oddities like an LPG injector or pipe leaking in such a way that it could affect the running when on petrol, but a failing coil pack or a duff spark plug is way more likely if it's only under load.
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)