No it doesn’t. The top line is the solo and the lower is the choir. So your image shows the choir dropping out and the solo continuing (you can check by looking at page view).
Just move the choral line down (paste special>move to staff below)
(Remember if you want playback, use Independent Voices and put the Solo in a reserved voice.)
Ah, I understand. So best to do then will be to make the Divisi at the beginning of the flow and don’t show the solo system until the last four bars..
I copied the Divisi signpost, pasted it to the first bar - and I had both systems visible.
Then I moved the Soprano choir music to the “gli altri” system.
[edit] After removing the Divisi signpost from the first bar, all the Choir soprano music vanished. So that didn’t work. I had to move the choir soprano music back up into the solo system - except the last four bars. Now it displays the way I expect it.
It seems a bit illogical to me that the tutti - up to the solo split - has to reside in the solo system.
Thanks, this is a longer piece, so I’ll try it next time 
The length is irrelevant, divisis can come and go many times and Dorico will handle the layout perfectly.
With any divisi, Dorico treats the top division as the main one, so it is consistent.
I think the visual representation in galley view is a bit misleading. It would maybe be clearer if the two divisi systems after the split would be displaced equally from the preceding unis system. However, whatever music is already in the system is regarded as belonging to the upper dívisi. But you have that figured out already 
It’s a dumb design decision that has been tripping people up ever since it was introduced.
I guess there is a logical explanation for it, and I remember, that I have read it somewhere.
It is the visualisation, which makes us trip over it, as we imagine a solo being above the group, like wind players being above the pond life (I mean string sections).. in a romantic orchestra setup, and as in a standard score setup of that type of ensemble.
The truth is, that a solo is also considered to be a part of a group, adding personal, empathetic comments to the musical event.
I have my personal take on this, as example, when it comes to performances of a Bach passion. When there is an aria, let us say with a gentle, expressive violin solo, this is what usually happens: they move to the front, they raise their music stands and they play it with an attitude of a Wieniawski concerto, which makes the listener forget how the main solo of the singer sounds like. I bet you that a professional instrumentalist can sing all instrumental introductions of the St. Matthew passion to you, but struggle when being asked, how the solo singers start.
If on tries to put oneself into the mindset of people of Bachs time, their religious way of thinking of the world and society, a solo aria would be a humble contribution or a very careful personal comment as an individual being part of a big community. I can’t really put this into good words, but somehow I feel, a composer like Bach, being very confident about his own abilities, would not expect instrumentalists put themselves in front of the sung word.
All that philosophy is no use when the current setup is actively inhibiting correct notation, like in this old thread for example. Don’t drag Bach into this.
I think, a good mnemonic can be: you have a choir, and the soloist is stepping to the front. If you look from above, the soloist is “underneath”.
Only if your feet are towards the audience! But that is beside the point. What is the accepted norm for a score, and does Dorico follow that as a default? (Of course it should allow exceptions.)
OT: I also think humility was not a part of most soloists’ perception of their ability. (If you really think about it, it takes a great amount of hubris to think you could compose something for “the glory of God” – as if the efforts of our imperfect human minds and hands could add anything to God’s inherent perfection and glory.) Just saying! 
– Jim
