When calling the function CXYStageBase.SetPositionUm(x, y) a Core Callback "OnXYStagePositionChanged(x, y) is automatically generated. This is confusing, since at the moment that callback is generated, the stage is not (yet) at the desired position. As a result, the UI jumps the position of the XY stage to the destination, then starts tracking the real location of the stage (if it uses callbacks or if the UI is polling.

I vaguely remember discussion about this behavior. I am now of the opinion this callback should be removed. It will be useful to add a function "boolean ReportsPosition()" or similar to indicate if the device adapter itself calls back to the core to reports its position, an if it does not, the UI will know that it should query the position of the stage.
@tlambert03 will that work for you?