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.
