If you bind a CheckBox with ControlSource
to a Field with a Check()/Trigger or a property
with a Assign,
then when the CheckBox.Value is put into the binded
value and Check()/Trigger/_Assign function change
the binded value, the CheckBox not reload the value
and you have CheckBox.Value#EVAL(checkBox.ControlSource).

WorkAround:

PROCEDURE checkBox.When
  RETURN
ENDPROC