Backing up a bit to your OP, the user field only stores the user ID in the database. It just returns a user object when you get the value. You can get the unformatted value of the field, an array of user IDs, by using the 3rd parameter in ACF $this_decision = get_field('decision_makers', $decision_id, false); I’m pretty sure that you can also just use an array of user IDs in update field.
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