Without going into this a great deal, what you are seeing is the difference between a cold pipe and a hot pipe.

Trust your first reading/pinch/needle setting where it jumps on the pipe.

On the racing engines (fan engines are very similar), we usually get one shot at seeing the peak RPM as the engine fully stages on the pipe. After that first approach, it will not get up there again (on the ground) with the hot muffler. If you try too hard to re-adjust, you will either overheat the engine, or end up with it too rich. Its tricky sometimes.

When the engine gets up on the pipe, it will increase in power, and require more fuel. You will normally have to launch rich to allow for that increased fuel flow. On most of my performance engines, I typically launch 600-800 rpm down from peak rpm. on the QM40 engines, they launch 1000 rpm down.

Sounds like what you have there is set up fairly well. As long as it throttles and transitions in the air, you have it set up very well. Do not let the transition on the ground 'fool you'. Keep in mind, the mixture will not change much from flight to flight. Once you have it set where you are happy with flight performance, remember that needle position. You will rarely have to move the needle more than 1/8 turn either way on any given day.