This has been discussed several times over the years: Arpeggios and accidentals - spacing - Dorico - Steinberg Forums. It’s particularly a problem with flats.
As a workaround, you could create a line with a wiggly body and use custom invisible Line Annotations (right-facing hooks with Line width = 0 for example) which you’d set as a Start or End cap for the line in the Properties panel.
Not ideal obviously but Dorico will allow room for the line so one doesn’t have to tinker with the spacing manually.