Some common reasons for not saving
ACF will only save to the acf-json folder of the current theme. For example you can’t write to the parent theme of a child theme unless you set up a custom save location. Also, ACF will not load json files from the parent theme unless a custom load point is added https://www.advancedcustomfields.com/resources/local-json/
This folder needs to have permissions set to that it can be written to.
If you are creating a theme that will use a child theme then the best practice would be to do all ACF work in the parent theme and set a custom load point to load json files from the parent theme.
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