Propane and gasoline have different burning characteristics that may make tuning a little interesting. Propane must be burned at the correct A/F ratio or combustion and/or exhaust temps can get out of hand. As you go richer with propane, it gets hotter. Since your burning both gasoline and propane you might need to change your target A/F ratio to burn the combination of both more efficiently. Its common knowledge that if you burn a propane fueled engine too rich, you will burn it up. Typically, a rich mixture of propane will increase head temperature and valve seat temps by 40-60 degrees F. How this knowledge relates to what your doing, I can't help you with that. I have no tuning experience with propane. But if the propane your injecting is burning in a rich mode, I can see where the extra heat might be pushing the engine into detonation. Also, propane engines need a low thermostat (165) to keep the engine cooler. Use at least one step colder plugs and close the gaps. Propane is harder to fire than gasoline. Propane is slow burning in lean mixtures and can increase exhaust temps and decrease valve life in that situation. If the propane is being injected as a gas, and is being mixed with the incoming air well, distribution would probably be OK. Try pointing the injection nozzle upstream to the incoming air for a better mix. If any of the content is injected as a liquid and converts to a gas in the intake track, I can definitely see the chance of a distribution problem. Again, point the nozzle upstream. A strictly propane fueled engine requires the ignition timing advanced below 1500 rpm and retarded above 2000 rpm. How this info works in your situation will have to be worked out. Put on your tune up hat. Good luck.
Another note. Propane contains less energy per weight compared to gasoline at a bit leaner A/F ratio compared to gasoline. I wouldn't get carried away injecting too much propane. It will just take up space that could be used to carry the higher energy fuel, gasoline. You might have more boost, but having more boost is only part of the equation.