(4Cyl TS Magna Dual Fuel)
My car recently suddenly developed a much deeper exhaust note, and almost simultaneously started playing up on LPG, dying, trouble starting etc, still 'fine' on petrol though.
I inspected and found a rust hole around where the two outlets leave the muffler.

Could the change in backpressure be causing this? Does LPG need to better tuned than petrol to run?

Or do I have 2 completely separate problems?

Any ideas?

My V6 TS running dual fuel has (at various times) had holes in its exhaust varying in location from the resonator all the way back and this has not caused any issues with dying or trouble starting.
I think you might have 2 separate issues (just bad luck that they happened at the same time).
You'll know soon enough when you fix the exhaust hole anyway.
Having said that, I get my LPG tuned and checked at rego every year and have never had any major issues with it.