So, I’m a little late here and you already figure it out.
The explanation is that when using update_field() you must supply the value as ACF stores the value. In this case you are using a user field and the value stored is an array of user IDs and not an array of user objects.
Most of the more advanced fields require you to know what ACF actually stores and like you discovered, you can figure out what ACF actually stores by getting the unformatted value by supplying the 3rd parameter in get_field(). Another way to do this is to search the database.
