Try a few degrees leaner needle setting. Just a tad. Even tho the tank is uniflo could be that the extra fuel head at the very beginning of the flight causes a slightly rich mixture. As the fuel head is reduced the engine might be running a bit leaner. The change may be enough to eliminate the burp. Wait. Wait. The inside loops precede the inside loop on the horizontal eight. Perhaps you turn the inside of the eight slightly tighter than the other insides in the pattern. Maybe a smidgen richer would work. I've had plane/engine combinations that worked well when the loops were 45 degrees but did not like 35 degrees. Recently I had a well used FP40 powered plane that did that. Swapped out the engine, replaced it with a $4 Tower 40 swap meet mutt, an engine that needed cleaning and unlocking before it would run. Plane and that engine are a sweet running combo. The Ro-Jett is, of course, a different class of power. Big bucks is big bucks. Doubt that you would want to simply swap that engine out and replace.
I thought synthetic lube didn't burn and leave varnish like castor. Would a lean run leave a residue anyway? Lean runs on AAC, ABC or ABN (how can I learn the alphabet this way) usually don't ruin p/l fit. The liner expanding faster than the piston until power falls off. At least that was what I thought happened. After the engine sits and cools down, clearances come back to normal, no damaging liner/piston gouges. Also no warped rings.
Is it possible that the back plate or cylinder head loosened up? This seems to happen to some of my engines. Dunno why. Especially when newly cleaned and assembled. An over lean run might also stretch those bolts or... whatever happens. I've been surprised a number of times by engines that need retorqueing now and then.