I think what’s happening is Dorico is interpreting “6s” as 6, plus a sharpened 3rd. If you enter “6s,” with the comma specifying that the preceding material is a “single entry”, you get #6 on a single line.
Ah! Found it. In note input options, there are settings for Interpretation of diminished intervals during figured bass input. If I set that to “Adjust from bass note”, I get the result I need – the 6s is interpreted as a sharp 6th, allowing the third to stay unaltered.