Editing attributes with multiple selection

[zargron]

Originally posted by kinook
Select all of the item(s) that you wish to add attribute(s) to. Ctrl+3, Ctrl+A to select all children/search results; Ctrl+1, Ctrl+A to select all siblings of the selected tree item; Ctrl+click to individually select/deselect item(s); etc.
Then go to Item Attributes and add the attribute(s). Ctrl+4, Insert, enter/select desired attribute and press Enter.

I've used this facility before. Today I had an interesting situation.
I had several contacts listed in the Child Items pane whom I wanted to put in the same "City". Some of the contacts were already in that city and the others weren't assigned any city. I selected ALL the child items and tried to insert the "City" attribute from the Item Attributes pane. However, the "City" attribute wasn't available. With experimentation I found that if I carefully selected only the items that didn't have a "City" assigned, I was then allowed to add the attribute. In other words, with multiple items selected, you cannot add an attribute unless no items in the selection have a value assigned to that attribute.
Kinook, is there a good reason for this or is it a bug?