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.
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