Assume the following interface and implementation:

public interface TheInterface{
    /**
     * This method does nothing
     * @param arg1 the argument
     */
    public void myMethod(String arg1, Long arg2);
}

public class TheImpl implements TheInterface{
    /**
     * @see TheInterface#
     */
    public void myMethod(final String arg1){        
    }
}

If the cursor in the javadoc of the implementation, directly after the # sign, I can hit ctrl-space to start autocompletion of the method name. This opens a list of the available methods. I select the correct one (myMethod) and the result is:

     * @see TheInterface#myMethod(java.lang.String)

This is what I expect, the arguments are filled with the correct type and the implementation javadoc is considered correct (since the javadoc of the interface is correct).

If instead I enter the first characters of the method to autocomplete, so that it is unambiguous and then start autocompletion, no list of available methods is displayed (since it is unambiguious), but the outcome is:

     * @see NewInterface#myMethod(null)

where the parameter is selected to be overwritten by the user. This isn't helpful here, since I want to autocomplete the declaration, not the call to this method.
There is no good reason why there are these two different behaviours on whether the method to autocomplete is ambiguous or not.