Actually, when working with repeaters and populating fields, I have not used the ACF api. I have not had to do work like this in some time and I have not figured out how to use it with with looping through or finding repeaters that have just been added.
The last time I did this what I needed to do what to delete all of the repeater fields, then I would add as many as I needed and then I would populate them all at one time.
This is only a guess, but I would say, create a loop that inserts on new row at a time. This insertion should trigger your append action and then the loop would add another field. I don’t know if that will work in practice.
