The systems on your last page are more widely spaced because there’s only 4 systems on that page, whereas on the previous page there are 5. You can either reduce the threshold for vertical justification to a level where Dorico doesn’t justify the last page - but this will probably end up with the staves more tightly spaced than the previous page, because the previous page is vertically justified.
Your other option is to select Frames in the Engrave mode toolbox, and move the bottom of the music frame on the last page upwards, until you’re happy with where the staves inside the frame are positioned. (If you want to re-use this page formatting, create a master page with the required music frame height so you can apply the same music frame size to multiple pages in multiple layouts.)