I came across this solution when I had about a ton of posts (200+) with values in various PDFs. I didn't want to have to spend time filling out the same values over and over again.
This case is pretty straight forward; I have one repeater field field_123a56b7cb890 that has a text fields inside it (field_588a24c3cb782). I haven't tried any other types like post objects or radio buttons, but I'm sure it can be done.
Step 1. Hook into acf/load_value on your repeater field. You can use type/key/name as desired here.
Step 2. Check the post_status to see if it is set to 'auto-draft', this is WP default when you create new post. If you want to override current values in posts you can change it to 'publish'
Step 3. Set $value to your array of default values. Above, I have 4 rows by default on new posts. Each post_type had different values so I check the post_type and add the values that way.
Step 4. Return the $value. Make sure you always return it – even if you haven’t modified it.