I noticed that, when I created a custom master page (to put page numbers on a First page) and asked Dorico to copy a LH master page to a RH master page, the second page was a copy rather than a mirror copy. (In both cases the page field I had added was on the left). In addition, the page text field was a linked duplicate: the text in both was justified left, and changing one field caused the justification to change in the other.
Perhaps @pianoleo’s situation is related to this.