If I select multiple nodes and then I do right click 'add keywords' the dialog works correctly exactly once... the following times I can't add a keyword already existing, only creating a new keyword. Logging out fixes it. For ONE time. Is this a known bug? (just like you can't add a category to multiple nodes?)
Please share with us some screenshots for better understanding about what is happening, because I'm not sure if I understood the issue.
Seem when you assign keywords to multiple documents, if keyword yet exists is not added ( that has sense ), but if some documents have keywords and others not, should add in the documents what they do not have, that should be the wished behaviour.