Ah – Of course I spoke too soon.
The actual problem is that you changed the left page expecting it to apply to the first page, but Dorico always puts odd-numbered pages on the right. The solution is to use the little L→R button at the top to copy to the right-hand page.
I notice this is complicated by the fact that you are displaying single pages vertically, rather than 2-page spreads. In that display mode the page template for page 1 really does appear to come second! (And I note that you can’t change the display while in Engrave mode! You have to go to Write mode.)