This may be a known issue that will be fixed in Dorico 4 etc, or I may also have done something very wrong; I’m not sure which. But I’m sure it has come up before.
I have part of a score in which the cellos are divided into two sections plus a solo player. At one point, the solo player rejoins the rest of their section for several bars over a system break, so I changed the divisi from Solo + 2 sections to just 2 sections. This is the result in the first system:
And in the second system—note that section 2 still has music in the first two bars, before both sections recombine in the third bar for unison double stops, but this music is not visible. (Nor does it become visible when I turn off condensing.)
In the first image, the now-unused solo staff is labelled 1 and the staff corresponding to the first divisi section is labelled 2, with the staff corresponding to the second divisi section not having a label; this is incorrect. In the second image, obviously, the second divisi section is missing entirely, despite having two bars of music. (The solo player also should have an automatic unison with the first divisi section, but I assume there is no way to make this happen at present.)
Is there a potential solution for this issue at the moment (and did I do something to cause it), or should I just retain the solo part over the system break?
When retaining the solo part over the system break, section 2 is visible for the two bars it plays. However, it does not condense or create an automatic unison when the divisi changes to solo + gli altri.
When manual condensing is applied (the only way to get the two cello sections to appear on the same staff), the gli altri indication is removed, as is the bracket indicating the use of a double stop, and neither of these can be reinstated.
It is certainly a known issue, and you didn’t really do anything wrong. Whether it will be fixed in v4 is anyone’s guess, though. One part of the problem is that Dorico can’t handle more than one change in the number of staves per system, so in your first screenshot it’s “set” to 3 staves at the start, and then the changeover only takes effect going into the next system. And then it happens in reverse: you start the system with 2 cello staves so the third one gets forgotten about.
Another part of the issue, which I in particular have been harping on about since divisi was introduced in the first place, is the fact that Solo staves replace Tutti staves, counterintuitively pushing everything else down instead of having Tutti I be the continuous stave, with any Solo staves appearing above it. In your case you are able to have clean divisi changes at either end of the solo (even though they obviously still go wrong here) but even when everything works as intended it always adds a layer of confusion and possibility of errors.
I’m afraid that for now, the only real solution is carefully coordinating system breaks with divisi changes, possibly duplicating some of the material so it shows up on the right stave. Obviously, when it’s time to make parts, or even when the score layout changes for some reason, this will turn into a headache to redo all these adjustments. Often I have to fork a project into a “score” and a “parts” file for exactly this reason.
