There were problems explaining this issue to devs during beta, and it's not been resolved for the live product.
With book layouts it is typical for the inner-margins, toward the spine, to be greater than the outer margins. Page element coordinates therefore need to be measured relative to spine, rather than fixed, or when you add a single page the wrong master is assigned for the new position of all pages after the addition. Breaking the book's layout, because pages that were recto are now verso.
InDesign handles this issue, and I understand most other professional DTPs do too. Affinity Publisher does not.
It now makes sense that alignment relative to spine was added so late in the beta c