Static RPM loss

Some thoughts;

Had the engine reached full running temperature for post service run up? (thick oil, less than optimum clearances, etc)

Wind direction & speed - relative to wind , which way was the aircraft facing?

Temperature- was there a significant difference in pre/post engine run ups? Changes in density altitude will effect engine/airfoil performance.

Air filter changes, if dirty - clean, if clean, may be too much oil used ( may improve over time).

Carburettor balance, if not done, should be.

Throttle cable binding. Common problem with Rotax 9 range - if old may be time to replace.

Propeller pitch - did your service provider do a static run? - if so may have adjusted prop to be closer to Rotax recommended Static 5200 RPM.