Indeed. The inner workings of how Dorico balances (particularly) the last couple of systems of each flow are complex and not always entirely predictable.
It’s not worth losing sleep over; you can always use “Make Into System” or the “Wait for next” property to tell Dorico exactly where you want System Breaks, and the chances are that once you’ve actually put some notes into those last eight bars Dorico will recalculate the casting off anyway.
It has its idiosyncrasies, sure, but I happen to rate its output (and speed with which to get there) above any competing software. No-one’s forcing you to use it, of course.
This isn’t a bug, it is deciding how to cast off the music using an algorithm, which is working correctly.
As pianoleo said the casting off will usually adjust itself as you add notes into those empty bars. Avoid forcing system breaks etc. until you see what Dorico does by default when all the music is entered. When you have many empty bars like that, Dorico does not know what your plans are and makes a best guess for how to cast off.
Just don’t go blaming it for those mid-directed stems. 
You could always work in Galley view until you have all of the music in place. Then see what Dorico does for the page layout.
Robby
For a long time I resisted Galley view, until I realized I kept getting distracted by layout issues - put everything in, then onto Page view/Engrave mode. Such a better work flow.
The more staves in your system, the more logical Galley View becomes.