The gear upshifts & downshifts are controlled electronically depending on road speed and engine load (throttle).
Later upshifts may be from a loss of engine power or drag.
Do you feel that you have to push more on the gas pedal to accelerate? Does the car feel like it is 'straining' more to get to speed? Any fault codes?
In a previous post, you had a 'whining on deceleration' concern. Is it still doing that?
You may want this issue road-tested with a technician for a diagnosis.
Always diagnose first.