Transpose key & notes at once?

I’m trying to transpose a leadsheet from Bm to Fm.
When “transpose key signature” is ON, I can’t select 5th diminished. So I have to do it in 2 steps.
Why is that?

Dorico limits the choices available in the Transpose dialog based on the selected music, so as to avoid coming up with a nonsensical result that produces pitches that cannot be notated. However, in music written using conventional 12-EDO accidentals, it’s rare for Dorico to encounter a combination of pitches/accidentals/key that would limit the intervals you can transpose (it’s much more common with e.g. 24-EDO music).