I resolved the problem, it seems that the problem was not the field itself – it was the acf-getallobjects add-on I was trying to implement. Just wanted to post it here so that people with similar problems might find a solution 🙂
But I am having this problem. I have posts that have ACF field where is user-type data stored. Before it was array, now I changed it to ID. When I am trying ti get_field, I don’t get id, I get an array. If I go to edit the post and save it without any changes, the value is presented as ID (and no longer as the faulty array).