Tuplets/triplets don’t seem to work with:
Write > Edit Duration > Double note duration.
As you can see here:
Please ignore the chords in the second image; I will enter them again manually as I already know they aren’t affected by the ‘double note duration’ tool.
I was doubling the tempo from 90 with swung 16ths to 180 with swung 8ths.
Thanks for your help.