This is NOT a backfire, but rather this is an afterfire.
All afterfires are caused by the exact same thing; namely a combustible mixture being lit off.
Eliminate any one of;
the fuel,
the oxygen, or
the ignition source,
and the afterfires should cease.
On decel with a programmable EFI, the easiest one to eliminate is probably the fuel.
On a carbed car, eliminating the oxygen is probably the first go-to.
On both, as to the ignition source, you want to start the fire in the chamber early enough on decel, so that the fuel/air charge has time to completely be consumed before the exhaust valve opens up. If the mixture is too lean, it may happen that the oxygen molecules are too far apart, and not all the fuel burns. This is called a missfire. Then when the exhaust valve opens, the header yanks the goofy charge out, and the hot exhaust valve can spark it up. Or maybe it doesn't and the O2 reads lean, and dumps more fuel in, which then accumulates in the system. The system is now under pressure relative to atmosphere, and the header may no longer be yanking from the head, but may instead yank atmosphere up the back side. And there is your bomb, ready to go off, all it needs is a spark or a hotspot, or a bit of flame inside the muffler, and boom!
The fire thus created will want to go up towards the head, but will likely run out of oxygen..... but the condition under decel if still existing, will continue to pump unburned fuel towards the muffler...... feeding the bomb until it runs out of oxygen..
Then it starts over, until the decel is quit.
Or if the header is still working and your cam has a lot of overlap, the header could be yanking air from the plenum, out into the primary pipe, which then mixes with the unburned fuel and voila there is your bomb from another angle.
This air could be coming from; the PCV, the idle-air bypass, the throttle valve opening, the AIS motor, even a ruptured diaphragm in the brake booster, etc. It all depends mostly on your sync, the pressure in the intake, and programming.
Sometimes more is less; that is to say a tiny bit more fuel on decel eliminates the missfire, and thus cleans up the exhaust, and thus, less afterfires or the elimination of them.
One thing occasionally missed is an intake sucking air at a port. Because you say the afterfires are only happening on the one bank, this is a real possibility. Check your plugs on that bank, maybe they have a story to tell.
