> So you'll have to manipulate org-map-continue-from appropriately.

Thanks a lot! Adding (setq org-map-continue-from (outline-next-heading)) after insert-string seemed to solve the problem.
(I added "newpage" to org-tags-exclude-from-inheritance, so the newpage does not get applied to subheadings)

(defun org/parse-headings-latex-newpage (backend)
  ; add \newpage to headings with :newpage: tag
  (if (member backend '(latex))
      (org-map-entries
       (lambda ()
         (progn
           (insert-string "#+LATEX: \\newpage\n")
           (setq org-map-continue-from (outline-next-heading))
           ))
       "+newpage"))
)

(add-hook 'org-export-before-parsing-hook 'org/parse-headings-latex-newpage)
(add-to-list 'org-tags-exclude-from-inheritance '"newpage")

Best,
Joon