Right now, this is expected behavior, because you cannot create new pages in a Figma file when in Dev mode. However, you can still see the page dividers you previously created in your Pages when in Dev mode.
@kaa_NS
Hi Karen, I was experiencing the same issue.
At the bottom of the list of pages, there is a grey button that says “Show all pages”. Clicking that button shows the dividers.
It’s weird UX. If would be nice to show the dividers by default.