Too buggy

ACF Photo Gallery field isn’t supported on ACF fields groups. There is a challenge when it comes to adding support to ACF fields groups.

I tried to give this plugin another chance, but… it’s even worse than I thought before.

It seams more or less ok when field data called on the page itself. But when it processed inside AJAX responce, for some reason this field’s data given as string instead of array.

called on the page: https://i.ibb.co/Wsywv7z/screenshot-01.png
called by AJAX: https://i.ibb.co/Vjb2sGd/screenshot-02.png

After having looked at both screenshots, it seems to me that you haven’t read the documentation for the ACF Photo Gallery Field. There is documentation, as well as video tutorials, on how to integrate this into your WordPress website. I noticed that your code does not contain any of the code outlined in the documentation.

Documentation: https://www.ads-software.com/plugins/navz-photo-gallery/
Video Tutorial: https://www.youtube.com/watch?v=5iTV0JVFFOE&list=PLHiFbddE22o87rwB_cewm-o-D44C54HLX

I have just now realized that this post is a review and not a support ticket. If you require further assistance from me, please submit a support ticket. At some point, WordPress will close this review for new replies.

So, why native ACF get_fileds() function works fine with your gallery field on the pages but fails in AJAX?

Your custom acf_photo_gallery() looks really poor optimized, simply unacceptably poor for the pages with lots of separated galleries.
https://i.ibb.co/vkJVcyL/screenshot-03.png