If this is a user error instead of a bug, I’m going to feel really dumb.
File attached.
Melodia Book 2.zip (464 KB)
I can reproduce the weird behaviour, but if I add a note to the bottom staff changing the clef then works normally.
Not divisi, just two good ol’ treble clef solo players.
There are more weird things here. Just as an experiment I dragged the second staff to the top in Setup mode, and got a rather novel looking brace. The clef problem still affects the bottom staff after swapping the staves, even though it’s not the same staff as it was before.
PS: this started its life as an XML import, long ago.
in a notation program far, far away…
HA! Quite so. 
For small projects I’d already done in that notation program far, far away, I’ll often choose to enter everything again in Dorico (it’s good practice, anyway) than to import it via XML. For a keyboard player, it goes really quickly and I avoid the anomalies. Large projects with ‘too many notes’ still get imported, though.
BTW, have you figured out what’s going on with your clefs? I remember occasionally getting Rob’s novel-looking braces at times, too.
That was this, sadly. About 2000 bars of music.
No solutions yet, although Leo’s advice worked: invoke the carat first. Still doesn’t explain the behavior, but catastrophe averted.
It’s MusicXML, Jim, but not as we know it 
Dan, I wonder why you still call the caret “carat”. No pun intended. Is there a reason? I know it can seem picky, but hey, we’re on a “notation program forum” here !
Oops. Carrot? It’s the right color. I’ll blame it on iOS autocorrect…
Ok, that’s a good reason. Now go and disable it 
The weird brace thing is because there was a manual bracket change. I bet if you delete that, the clef will render correctly too. I just ran up agains the bracketing glitch a few days ago and emailed Daniel about it. Essentially, Dorico doesn’t delete custom bracketing changes when you change the order of staves (this was a manual override after all) and therefore it can’t figure out how to properly draw the brace. This glitch caused the barlines to disappear in one of my 3 stave organ parts (pedals only; manuals were fine). Deleting the custom bracket change took care of it.
I spotted the bracket change, but deleting it didn’t fix the clef change problem for me. So I assumed it was irrelevant to that.
The reason this is behaving strangely is because multi-bar rests are enabled, on the Players page of Layout Options. When you select a bar rest when multi-bar rests are enabled, you’re selecting a different kind of rest than you see when multi-bar rests are not enabled, and for various boring reasons it cannot differentiate between the multi-bar rests on the two staves in the layout, so you end up creating the item on the first staff rather than the second. Provided you have a “real” item selected, or the caret shown, then Dorico can correctly determine where the item should be created. If you don’t need multi-bar rests on for this layout, switch them off, and that will eliminate the problem altogether.
Aha. Thanks Daniel.