See attached files.
Between bar 23 and b25 of the soprano part there is a divisi, though not an obvious one: I added a soloist, then removed the ‘gli altri’ voice because there is nothing for ‘gli altri’ to do here. Thus although there is a divisi, there is still only one soprano staff.
The part of Dorico that is responsible for adding the dividing and reuniting arrows at the ends of vocal staves does not deal correctly with this situation. So at the end of the first system on page 2 there are dividing arrows although there is still only one soprano staff on the second system: Dorico evidently thinks there are two.
On page 3 we have the converse of the same bug: required dividing arrows are missing at the end of the first system although sopranos are divided in the second system, because Dorico thinks there are already two soprano staves in the first system.
It’s an obscure little bug, but it makes these quite useful arrows unusable in a piece with solos.
I’m not sure if this really needs to be divisi, if both players never play simultaneously.
Maybe the simplest solution is to just use text to indicate solo and tutti, if you’re not concerned about playback?
I also thought about including the soloist as an instrument change; unfortunately I just tried this and it seems like you can’t add a solo player to a section, and vice versa. Maybe another possibility is to add a soprano soloist in Setup mode, and hide empty staves, although this would create extra staves in some systems.
That is so — the problem can be worked around if playback isn’t an issue — but it remains a bug with the divisi tool, which I’m reporting.
It’s not really a bug, for what it’s worth, though perhaps it indicates a small deficiency in the design of the feature. The arrows will appear when the number of divisions changes from one to two or from two to one, but it doesn’t consider whether one of those divisions will not appear.
I’d suggest the feature should consider the number of divisions that actually appear in the score (one vs. two or more). To singers reading the score, the arrows indicate that a staff divides or divided staves reunite, and if they don’t the arrows are meaningless or misleading.