Sounds like detonation
I too had this same problem a while ago with my Sirio .21 PR-Pro on road engine. Everything would be running perfectly and when I would lift at the end of a long straight, the engine would "run-on" for a few seconds then settle back to normal idle.
After playing with the mixtures, I could never completely correct it.
What I found was the engine was detonating and that was causing the run on condition. I ended up putting .05mm shim and one step colder plug and the problem went away with no change in performance.
Pull the head of the suspect engine and look at the top of the piston and the surface of the head button for minute pits, if they exist then I would say the problem is definitly being caused by detonation. And a shim or two should fix your problem.
Remember to reset the mixtures after you shim the head button.
p.s. I agree, the low end mixture is also a bit lean, which is adding to the engines tendancy to detonate just after closing the throttle.
Hope this helps.
TLR Design Engineer
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