Hotlanding,

-Try to run the engine warm before flipping the plane. Maybe a minute or so on medium throttle.
-Also try to remain 1/3 rpm, lets say 4-5000 rpm when flipping.
-If that does not help, try changing the plug

It should not stop. If it stops when turning it, it would not function in air either.

One last installation suggestion: Check the pressure line's pipe in the tank, that it points upwards when the plane is up right. If it points down you can get back pressure when releasing the gas, which can make fuel pour back to the silencer. The silencer builds an over pressure in the tank which has to go somewhere. In that case the silencer fills with fuel, which most of it blows out, but what's left may pour into the cylinder after stopping the engine. Also If the piping is correct installed, you may need slightly less needle opening when plane is up right due to the above statement of back pressure. In other words screw the needle valve in a little before flipping the plane.

One other suggestion that I had to do with my heli that had the tuned pipe mounted higher than the cylinder, hence fuel ran down after landing. I removed the plug and ran the starter for a couple of seconds. That cleared the lock.

/Fredrik