Unfortunately the reasoning behind the extra staves is beyond me, but it looks like it’s something to do with the multi-bar rests between bars 357-367 (if you add system breaks in various places, you can narrow it down to those bars only appearing with 3 staves). Edit - shortening the oboe cue as Leo suggests does indeed seem to work.
On a side note, and apologies in advance for the tangent - I couldn’t help but notice you’re doing a lot of your page formatting using local page overrides rather than editing the underlying master pages or creating custom master pages, e.g. for the title page in the score and p1 insert for the parts. I really would recommend doing that as much as possible, as it “saves” your pages more safely than as local page overrides and in the case of the parts, might save you drawing in the graphics frame on every page 1! (here you’d just need to load the relevant image into the Default Part master page for it to appear everywhere) You can still of course override that master page in each layout, e.g. to tweak the size of the graphics frame, but it means that if you ever need to reset pages, they get reset to a fuller baseline, as it were.
You don’t have to use the same master page set for portrait and landscape parts, if you want those to be different.
You can now also export/import master page sets between projects, which means if you have a preferred title page template, you can reuse it in multiple projects (and if most of the detail comes from tokens, it might not even require local editing in each project either).
Here’s your project with a couple of tweaks in that area, including applying the relevant paragraph styles to different bits of text with their formatting coming from the Paragraph Styles dialog (although I probably don’t have the exact font you were using, so if you wanted to work from this project you’d want to change that for the Title paragraph style). Condensing player labels are a bit further to the left by default, so they don’t contribute to vertical stacks so much.
I also made tempo mark and rehearsal mark font styles absolute, so they’re larger in the score. Ditto score bar numbers. And adjusted staff spacing (reducing minimum gaps between staves in the score to give Dorico more flexibility in allocating space for staves with extra info; reducing system gaps in some parts to make more staves fit automatically on pages without distortions that come when the page is calculated as overfull).
To hide flow headings in single-flow projects, see here. You can see I’ve added the subtitle token to the First master page in the parts instead of using the flow heading for this.
5 Resilience Concert Band TEM_LH.dorico (2.1 MB)