I have been doing some testing. Load field is only called once which explains why the value is not changing for you. The reason I suggested it is that at one time load field was called for each instance, at least I think I remember it that way.
However, I did notice while testing that if I set an ID for a sub field of a repeater then that ID is used multiple times. I think that is a bug, the ID value should be appended with the row by ACF, at least I think so. I have submitted an bug report for this.
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