As far as I’ve observed, this particular issue is that the key signature state of a divisi line (including a unison range expressed in that divisi line) can occasionally get stuck in a previous divisi’s key signature. Below is an example of missing necessary accidentals for C♯s and F♯ both in the unison range before the divisi and after the divisi, where the previous two-line divisi I had a page earlier was in two sharps. I imagine that is what’s going on here and is probably the case in your example, too.
(Since someone will ask, the second divisi flag is to disable group names on subsequent systems. I have tested to make sure it’s not related)
Here’s an example of an unnecessary cautionary accidental — again I believe it thinks it’s still in two sharps and thus thinks it’s quite necessary.
I have spent an hour rewriting this post trying to find an exact rhyme or reason to when and where this happens, but I can’t find it myself. Daniel is right that at least for now the best way to deal with this issue is layout-based — either moving the divisi flag somewhere or changing a system break. I don’t know why that corrects this issue, but manually “reminding” Dorico of the intended key signature state of accidentals does not work after a save and close.
An additional thing I’ll say is I am unable to reproduce these kinds of accidental issues in new projects. It seems to only happen once you get into larger projects, perhaps either due to more flows, or more divisi flags? I can’t say.