This should have nothing to do with blocks. Your field name is likely changing because it is cloned and may sometimes be a sub field and sometimes not.
I would use the generic filter hook acf/fields/post_object/result (no key or name). and then check the field name in my filter something like
if (strpos($field['name'], 'download_post_object') === false) {
return;
}
You may also be able to do this by using the field key variant, but depending on how the field is cloned the field key might also be changed when ACF runs these filters, I’m not sure.
