A possible work round:
Create an empty triplet (containing just rests). Select the tuplet bracket and press R a few times to duplicate it. You can then enter the notes with the mouse in any order you like.
FWIW I discovered a more “logical” way (at least from the way my brain works) to enter a single tuplet from the keyboard: press ;, enter the ratio in the popover, then immediately press :. The : doesn’t take effect till you reach the end of the tuplet.
The way my brain works, I know I only want to enter one tuplet when I start entering it, but by the time I reach the end of it I have often “forgotten” that I have to cancel it, and then switch my brain from “typing notes mode” into “typing general commands mode” to press ; and then switch back to “typing notes” again.