If I ever needed to use a lot of throttle the system would always switch over to petrol, ... I presumed it was normal but maybe not, the installers said it was when I asked them.
They fibbed. It is a consequence of under-specifying somewhere in the hardware. It can be difficult/expensive to provide a full-spec system on big engines. Customers tend to go for the cheapest quote (as everyone says their system is the best), so it is a "common" symptom in this world, but not "desirable".
The times when the system has packed up in Europe it will typically be on a motorway, running on cruise control at 130Km so not heavy throttle. Quite often it did seem to happen when the motorway was climbing slightly so the load would have been higher.
That sounds like reducer icing under sustained load (as opposed to traffic-light racing). Could be poorly laid out water plumbing, could be simply underspec hardware again.
The fact that the last time the system was fixed it needed new LPG injectors, would this indicate they are running too close to maximum / not getting enough LPG ?
No. The hammering that injectors get has little to do with load or stress.
Some simply last longer than others. Very few have anything like the longevity of petrol or diesel injectors, refer back to "cheapest quote" etc. which applies everywhere in the manufacture/supply chain.