im pretty sure ( i dont use keypoints for construction) the behaivor of the Circle is what you are looking for. its the only keypoint “arc” that the radius is based on the center NOT the sweep inputs.
Surface Geek stuff.
im very doubtful that your surface is moving. trimming does not rebuild or reinterpret the shape. what you are possibly seeing is the Draw Precision of the surface change after the trim. try this. CONTROL PANEL > QUALITY > DRAW PRECISION set this to 1 and see if the shape looks better. my fingers are crossed for ya. if so, everything displayed in a computer is a polygon or polyline. after you trim, the flag segment might be showing right where you trimmed. setting this to 1 is the maximum number of segments to draw a shape. set it to zero you will see the segments very clearly. BTW: this is not set to 1 by default because it will slow the display down especially for super dense models.
i could get into tolerances if you need but i would first start with the display.
Masochistic. Thanks: ) but i did not model this head. it is one of the human models that came on the 3DCD (if anyone remembers that) once upon a day, Alias was called Power Animator and was a real hit in Hollywood hence all the animation tools still in the software. if you want to see some of the first uses of Alias, check out Abyss(the water creature) and im pretty sure Terminator 2 (the chrome guy).