In the attached sample, to get the shown result the interval should be 2, not 1.
Showing accidentals on trills is not yet fully implemented. It should be the case that Dorico would show the appropriate accidental without needing to change any properties, but at the moment it is implemented in a half-baked fashion, so 1 = flat, 2 = natural, 3 = sharp. When properly implemented, Dorico will show the correct accidental, take it into account in the music in the remainder of the bar, and also show the appropriate thing when viewing in either concert or transposed pitch.
Ok, fine, thank you.