I haven’t personally tried filtering a message field, however your syntax looks slightly incorrect.
See this page: https://www.advancedcustomfields.com/resources/acf-load_value/
Specifically:
// acf/load_value/name={$field_name} - filter for a specific value load based on it's field name
add_filter('acf/load_value/name=my_select', 'my_acf_load_value', 10, 3);
// acf/load_value/key={$field_key} - filter for a specific field based on it's name
add_filter('acf/load_value/key=field_508a263b40457', 'my_acf_load_value', 10, 3);
You have name in your code where you are passing a field key.
