Okay, that seems to work with a very simply repeater field.
Try this. Create a Group. Within that group, create a repeater with 3 fields.
Create a post in the admin area as normal.
Use the FIELD_ID and the POST_ID in get_field() to get the data for that group. You should get a nested array.
Modify that array and use update_field() to put the data back. It should work just fine.
Now, wp_insert_post() to create a post. Use get_field() to get the data from the first post with valid data. Use update_field() to copy that data into the new post. If fails.
