AUCTeX: inconsistent behaviour when compling (probably becuase of what I'm doing)

Question

I cannot get AUCTeX to preview updates to multifile documents.

I have the following config:

 (use-package tex
    :ensure auctex
    :config
    (setq-default TeX-master nil)
    (setq TeX-source-correlate-start-server t
          TeX-source-correlate-method 'synctex)
    (setq TeX-auto-save t
          TeX-parse-self t)

    (add-hook 'LaTeX-mode-hook
              (lambda ()
                (turn-on-reftex)
                (setq reftex-plug-into-AUCTeX t)
                (reftex-isearch-minor-mode)))

    ;; Needed to sync TeX and PDF
    (add-hook 'LaTeX-mode-hook
              '(lambda ()
                 (TeX-source-correlate-mode 1)))
    (add-hook 'TeX-after-compilation-finished-functions
              #'TeX-revert-document-buffer))

  (use-package company-auctex
    :ensure t
    :init (company-auctex-init))

  (use-package reftex
    :ensure t
    :defer t
    :config
    (setq reftex-cite-prompt-optional-args t))

I preview the documents in my external PDF viewer (PDF Expert). This sometimes works well with single documents (like the circ.tex doc included with AUCTeX). But I could not get the same behaviour with multifile documents. Now, I cannot get it work at all! Other potentially relevant info:

ProductName:    Mac OS X
ProductVersion: 10.14.6
BuildVersion:   18G3020
Emacs: 28.0.50
TeX 3.14159265 (TeX Live 2019) (installed via TinyTex, see https://yihui.org/tinytex/)
ghostscript: stable 9.50 (installed via homebrew)

I can build and compile the documents externally or from R using tinytex, however cannot build from AUCTeX.

What am I doing wrong? Am I missing a package? Am I missing a step in the process? Do I have an incorrect config? How do I get this to work?

Please let me know if you need any more info about my request.