I don’t think I’ve even seen this before. In a nutshell, notes disappear.
Galley:
Engrave:
The project:
condensed-divis.dorico (1.2 MB)
IIRC there is a limitation that you can only condense one divisi change in any system.
(but I may be misremembering)
Does that mean with the above div. a2 → unis → div. a3 that the div. a3 won’t condense? Or, that I can’t have div. a2 and div. a3 in the same system? Regardless, some kind of warning would be warranted. Just dropping notes isn’t very graceful.
There are threads discussing this.
Missing notes without a warning message is a critical design flaw in Dorico, but the team is aware of the issue. For now, you have to make sure that your div. a 3 is in a new system.
It’s a bit more nuanced: you can have as many changes between 2-way divisi and unison within a system as you want. Or between 3-way and unison, for that matter. If you start a system 3-way and later go to 2-way, the third staff will have rests. (But Ends Voice does work to make those into a blank staff.)
But if the first divisi on a given system is 2-way followed by 3-way (or more generally: n-way followed by m-way where m > n > 1) the bottom staff/staves will disappear, regardless of condensing. With experience, you learn to flag these spots when they come up and work around them but this is still definitely something that should be fixed at some point.