Dorico seems to have trouble positioning a portato in the following situation: the portato is on the note head side of the outside voice and an inside voice overlaps the first rhythmically as seen at X. There is no issue with a normal slur or if there is no rhythmic overlap as seen at Y:
This kind of situation, where a slur is attached to a note in the middle of a chord (even a chord made up of notes in multiple voices), makes Dorico consider the slur as a so-called “inner slur”, in which case no collision avoidance is performed for the slur.
Thank you @dspreadbury I tried some other situations and collision avoidance would be very difficult to carry out.