When I select anything from a single note to several measures of notes (I tried several different areas to see if there was a workaround) to convert to rhythmic slashes, Dorico replaces all of the notes in the score with rhythmic slashes.
If you go Edit > Voices > Rhythmic Slashes, you are changing the way that that entire voice is displayed.
You want Edit (or right click) > Voices > Change Voice > New Slashed Up-Stem Voice.