At present one must press the "search next" button after replacing in instance.
(Unfortunately this is not always the case. Sometimes it is jumped to the next instance after replacing. So far I couldn't find out under which conditions it is done when not.)
In -- I would say -- every program I know with the functionality of search/replace it is jumped directly to the next instance of the searched expression after replacing an item. Could you implement this for Mathcad Prime, too?