I have no idea why this sometimes happens, but for whatever it’s worth, I’ve noticed this very often in Dorico 2. I always assumed it was because I was doing some pre-Dorico3-condensing hacks, i.e. putting two flutes on one stave rather than having separate instruments in setup and galley view.
I also have no idea why, when you have sub-brackets set for like instruments, you get this on violins and horns, but not in other places. (I prefer sub-brackets on all woodwind doubles, personally, so for instance piccolos and flutes sub-bracketed together. I frequently do this manually in Engrave mode; the change usually sticks on subsequent pages until an instrument change is made, at which point I have to pop into Engrave again. Then again, I too spend very little time in Engrave mode (so far sub-bracketing is the main use case for me), and it’s very possible I’ve set things up improperly and may be creating a bit more work for myself here than necessary.)
When doing multiple divisi staves on the strings, sub-brackets always get jacked upon return to unison, in my experience. So for instance, when adding a divisi stave, Violins I and II are frequently still sub-bracketed together, even though it would frequently be visually less confusing to only sub-bracket the divisi staves, and upon return to unison (even with no manual changes in Engrave), Violins I and Violins II suddenly each get a sub-bracket of their own, rather than returning to being sub-bracketed together. It ain’t hard to fix this in Engrave mode, but it would be nice if we never had to, or at least if it was a lot less often (I divisi frequently).
You mentioned that creating new instruments results in proper bracketing. Here’s another little quirk I noticed when playing with your file: If you just select for instance the bassoon card, and then “change instrument” to bassoon (as nonsensical as that sounds), it brackets properly. That’s what I did in the attached example; not sure if it’s any quicker/easier than your hack. Note: this doesn’t seem to work on the percussion; for that I know of no way except manually in Engrave, or possibly using your create-a-new-instrument hack, which I haven’t tried but will keep in mind for the future.
whacked_brackets.dorico.zip (745 KB)