Absolutely wrong. With any duel fuel LPG conversion it should start and run exactly the same on petrol as it did before it was converted. The most common faults with petrol fuel systems when on LPG are those of disuse. Blocked fuel filter or partially blocked injectors, dying fuel pump, water (condensation) in the petrol etc.
It's also possible that the petrol only MemCal has been retained. When running on LPG the poor thing will be trying to learn corrections for the air:fuel ratio using the exhaust gas from the LPG mixtures. If the LPG mixtures are not at stoichiometry, sometimes deliberately set so, the petrol mixtures will be wrong when initially run on petrol. It will relearn but that can take a minute or two. You can either drive around on petrol for a few minutes or clear the ECM memory (remove the fusible link closest to the passenger side of the car in front of the fuse box in the engine bay for >10 sec). If the car then starts OK, you've diagnosed the problem; the LPG system should be retuned.