$repeater = get_field('contributors_grid','option');
// try echoing it to see it it's an array
There are times when this might return an array, but there are other times when it will just return the number of rows in the repeater. It is unclear to me why there is inconsistent behavior like this when getting a repeater.
You may need to loop through the repeater to get the values before you can randomize them.
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