Welcome to the forum, moonfruits.
These Layout Options are the rhyme and the reason:
If you click the Staff Spacing icon in the left panel of Engrave mode (the third one down at the far left) you’ll see a percentage at the bottom of each page. Your left page is over 60% full, and your right page is under 60% full.
As per the Layout Options above, Dorico justifies the distances between staves and systems when the frame (page) is at least 60% full, and it doesn’t when the frame (page) is less than 60% full.
For further details please see vertical justification 100 % doesn't fill the first page? - #10 by dansoper