Not sure why you made your comment private.
Like I said, I don’t know anything about building custom walkers and I don’t know why it would not be working.
As far as my knowledge goes, when using a select field for taxonomy terms is that ACF calls wp_list_categories() and one of the filterable arguments for this function is ‘walker’.
The best I can do is tell you how to see if your filter is being called. For this turn on WP Error Logging and then call the PHP function error_log() inside of your filter and see if your messages is logged. This is the only way to tell because it is a AJAX request. Your filter will only run when you click into the field and ACF tries to load more choices.
If you want to possibly get input from someone other than me then you need to make your comment public, but it reality, your best choice for information on building a custom walker for WP is probably not this forum.
