You cannot generate different values for defaults, or any other value/setting of a sub field. The code for loading the sub field is run just once, each row will have the same value and this value is determined by what ACF puts in a hidden row in the repeater that it uses to copy it when a new row is created. There isn’t a way to do what you want for each row, unless you maybe use some custom JavaScript. https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/, in fact, I think this is the only way to achieve what you’re looking for.
