Solved it. (guess I forgot the page lifecycle stuff)

Changed my auto property to set the control value as soon it's value changes.
Code:
        private string _workDateId;

        public string WorkDateId
        {
            get { return _workDateId; }
            set
            {
                _workDateId = value;
                workDateId.Value = _workDateId;
            }
        }
Okey Dokey now. (and posting next stupid one soon ;-) )