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Backfire in muffler after fuel stop

Rather than extoll on the benefits of Seafoam, there are Youtube videos on the "decarboning" procedure which is basicall running a mixture of Seafoam, some dilute with a littel water, and spraying it through the intake. The process usually generates a fair amount of smoke; I have seen chunks of carbon fly out the exhaust.

I have also seen cranks tweaked by a nasty kickback on a heat soaked restart. Keep us posted.
Found this on the old internet. Could also be a timing issue.

The most common cause of a motorcycle backfire is unburned fuel igniting in the exhaust system. This usually happens because the spark plugs are not firing correctly. If your motorcycle's spark plugs are old or degraded, they may not be able to create a strong enough spark to ignite all of the fuel in the cylinders.

All the comments are good suggestions and may very well be what is causing your issue.
Good luck and give us an update.
Backfire in the muffler? I am thinking the backfire is in the combustion chamber and presents in the exhaust. Is there a "kick back" associated with the backfire? Spitballing here, since the OP has addressed all other possible sources. At 136K miles, if the head breathers have not been vented to the atmosphere, there is probably a fair amount of carbon build up on the tops of the pistons. A bit of fuel could be ignited by a carbon "hot spot" which is causing the backfire? Maybe running some Seafoam through the motor to "decarbon" the piston tops would eliminate the "hot spot"? Like I said, spitballing.;)
I am pretty much convinced that you hit the nail on the head, Amigo!
I didn’t want to use the spray-in SeaFoam, but I did add some to four tanks of gas, about 1-2 oz/ gal concentration. And I ran the thing pretty hard-high RPMs. Almost immediately, I felt like it had more power, and it is wayyyy better about the ker-pow when starting warm engine. I’m going to continue to treat my fuel going forward.
I also added 4 oz of SeaFoam to the engine oil around 300 miles before an oil change. It seems to have quieted the lifters and vavles.