When I highlight an existing eighth note and to create a 3:2 tuplet (eighth-note triplet), Dorico also grabs whatever is in the next two eighth-note positions and coopts this into the tuplet in order to complete it.
From:
To:
In this case, is there a way I can highlight just the first eighth note, press “;” and then enter “3:2”, and only affect the music in this one beat without changing anything on the second beat?
It took me overnight but now I remember why this has changed: Other times users have complained about the opposite problem, that Dorico ignored the gap after notes when creating a tuplet in Insert Mode.
So now Insert Mode has new features and behaves more consistently: For the OP’s desired result, select the 16th rest, type Shift-Alt-I to set the stop position, then create the triplet, and the following notes are protected from being subsumed.