Your initial screenshot appears to show a unison melody? If they’re not automatically being put into one voice, that perhaps suggests there is a subtle difference somewhere in the phrase - and Dorico considers “phrases” to be anything and everything between rests. Different dynamics, slurs, articulations all count as differences.
Adding a condensing change and including that group on the top staff in it (even if you don’t change any of the options in the condensing change, other than activating the group) at the position where the parts are no longer in exact unison should restart phrasing at that position, and allow Dorico to recalculate this first phrase. (As Leo has said of course)
You can force a unison using manual condensing, of course, by putting all the parts into the same up-stem voice on the staff.