A big issue is that you cannot specify multiple locations to upload a file to. ! file can only be uploaded to 1 location. You will only change the upload location to the last user in the loop.
And yes. The file is uploaded in AJAX and will be uploaded before the value of the ACF field is updated.
In this case you would need to use an acf/save_post filter, get the users and then duplicate the uploaded image for each user….. I do not know how you would do this duplication.
