Exhaust Backfiring

Snowmobiles do that too. And certain 4-stroke motorcycles. And occasional quads. And once in a blue moon an outboard engine with a coil going down. The point is, it is almost common, in that world.
I mean after the plugs get fouled,maybe even just once, then they are trash. They might look almost new, but the two-bit ignition systems can't deal with it. It used to be a common thing to tell customers booking appointments with misfires, to put all new plugs in it, and roadtest it. I was always backlogged weeks to months, and didn't have time to deal with these kinds of almost trivial issues, and had no desire to lighten their pockets for a plug swap. Invariably the plug change solved the issue. And I won a customer for the major work.

In the automotive world, before EFI,stock coils in stock applications,had pretty good power, and could usually spark anything.As compression and power goes up, sometimes the coils can't keep up. So they invented plugs that were easier to fire, and, occasionally run smaller gaps to make it easier on the coil.Then they added exotic materials to help them last longer.And then we got COP;coil on plug.
So here's my 2cents; go back to a copper plug, or make sure you have a decent coil,(I run the Big Yellow Accell,with .045 gaps)or the problem is likely to re-occur. And find out how air is getting into your pipes and fix it.The popping is from an unburned fuel charge(s) in the pipe,reacting with clean air, being ignited from a following, still-burning charge. It ignites in the pipe and then you get a pop!, as the expanding gas rushes down the pipe. The expanding gas may go in both directions.I suppose with a good amount of overlap, and the right engine rpm, the pulse could even get into the intake.
If one header pipe is sucking air at the head, then it won't scavenge properly,and that means that cylinder will be down on power a bit. If the collector is sucking air, that would probably reduce the strength of the return vacuum pulse, and the engine will be down on power, on that whole side; but probably not near as much as a flange leak. Flange leaks are to be avoided like the plague,almost.
Happy hunting!