This is where I generally bypass the pretty fields in ACF like post object and relationship fields and instead I opt for dynamically populated select fields. 99% of the time when I use a post object or relationship field I only get the post IDs and don’t use the built in ability of WP to get the actual posts and do all the other work myself. The, with the select fields I create my own AJAX requests through ACF to get the information for other fields. I have some examples of this here https://github.com/Hube2/acf-dynamic-ajax-select-example. I just built a similar set of fields where one select field is only parent terms in a taxonomy and the second select field is dynamically populated with only the child terms of the parent term selected. Might this be possible with ACF alone? Maybe, but it took me a whole lot less time to build it myself than it would have to figure out the hoops I’d have to jump through to use ACF taxonomy fields. I don’t know if that helps you at all.
