I ran into this last night working on a mass. Very strange…
File with instructions is attached.
Triplet bug.dorico.zip (252 KB)
Edit. Sorry. Wrong assumption form my side.
It’s not a bug, and it’s not strange. When you click the triplet button (or type ; and then 3 into the popover) Dorico creates rests to give you room for three triplet notes of the value you’ve selected. Because you’ve selected a quarter note rest, the note value that’s selected by default is a quarter note, so Dorico gives you space for three triplet quarter notes. In this instance you’ve got a barline in the way so Dorico’s forced to give you six eighth-note rests, rather than three quarter-note rests.
If after selecting the quarter note rest you’d selected the eighth note from the note-values (or pressed 5 on your keyboard) and THEN clicked the triplet button, it would give you three triplet eighth rests.
I see… You’re right. My mistake.
Thanks for the clarification.
Dear Rerei,
You might want to change the title of this thread (maybe with a[solved] marking and more explicit details) in order to make it easier for new users to access it using the search function…
Dear Marc,
Done. 
Thanks, ReRei ! 