new page has different Master Page design

To make the third flow, titled “3. Allegro”, I imported XML same as the previous two flows. This time my Master Page formatting disappeared.
How do I fix that?

and how can I make the staves further apart in the “3. Allegro” flow without affecting the other two flows?

Your first flow is presumably using the First Master Page. Unless you’ve told Layout Options to use the First Master Page for any flow that starts at the top of the page, it won’t - you must have done something (either inserted a master page change or dragged things around manually) to make the second flow use/appear to use the First Master Page.

Your screenshot only really tells us what you don’t want; it doesn’t tell us what you do want. Either upload more screenshots or (preferably) upload the project.

The quickest way to level out the staff spacing in the third flow is probably to stick a Frame Break at the start of the (currently) final system on the first page of the third flow. Then on the second page of the third flow, switch to the Frame Editing submode of Engrave mode and drag the bottom of the music frame up so that it only fills half the page.

I would like to send my project for you to look at, but the dialog for attaching files to replies grays out .dorico files. What should I do?

Zip it first.

Right. I thought of that on my way out the door. Back now, here it is:

It’s a mess, I know. This part is hard.

… or even just change the .dorico file extension to .zip

Same file zipped again. Didn’t see it on the post.
Prelude, Fugue and Allegro BWV (1.19 MB)

It looks as though you haven’t actually edited the Master Pages at all - you’ve manually added some frames to individual pages.
Edit the Default Master Page by double-clicking it in the right panel of Engrave mode.

That’s where you need to add the extra frames. You can then right-click on any of the pages with red corners, in the top of that same panel, and click Remove All Page Overrides. Your pages will then all respect the choices you’ve made in the Master Page Editor.

The screenshot that this project has all my Master Page work done and available. When I do the next piece, I want the same Master Page work to be there exactly as I left it in the previous one. In other words, I want all my projects to look the same. Is that possible, and if so, how do I do it?

You can either duplicate this project and import flows into it (or delete the existing flows and start again with new ones), or you can export the Master Page Sets and import them into your other projects.

I must emphasise that in the project you uploaded yesterday you had not actually edited the Master Pages at all - you’d moved and added some frames on individual pages. Are you sure that the changes you’ve now made are on the Master Pages?