Hi,
I think this might be becasue the values are Defaults - once a value has been selected for the parameter value the default will not be updated as a selection has been made that overrides it.
I have recreated this problem - the default value shown for my dependant parameter is updated based on the selection for my first parameter but once I have selected a value for the second parameter changes to the first parameter will update the available values for the second parameter but not the default.
This seems to be correct behaviour to me - once a value is picked, a default is not appropriate.