There are many scenarios for using acf-json with parent/child themes, and this depends on whether or not they should be editable in the child them. Honestly, going into every use case would be impossible on this forum.
However, once a field group is edited in a child theme then you cannot make any changes to that group in the parent them that will affect child theme sites where changes have been made. In most cases this is not going to be the desired use case since the purpose of having a parent theme is to be able to make changes that affect all sites using a child theme.
If you are already at a point where changes have been made to sites using the child theme and you want to make changes to those groups in the parent them then you likely have a problem than cannot be resolved easily.
You can force the child theme to use the parent theme json files using the code I posted in the other topic, but to do this you need to use array_unshift() to push the path to the beginning of the paths array so those groups are loaded first. Any groups that are loaded from one path will not override or modify groups with the same field key loaded from previous paths.
