Instrument changes work best if they occur at barlines. If you have a transition that needs to happen within the space of a single bar, then you should switch to galley view and ensure that you have explicit rests (created by switching on Force Duration, i.e. shortcut O) on the staff belonging to the instrument that you are switching to in the same positions as the rests on the instrument you are switching from. For example, if your from-instrument ends after an eighth note at the start of the bar and shows an eighth rest followed by a quarter rest, then you need to create identical rests on the to-instrument, i.e. two explicit eighth rests followed by an explicit quarter rest. When you switch back to page view, you’ll find that the missing rests are no longer missing, because Dorico’s now showing the rests from the to-instrument.