I using the web interface to do this (Chrome 32.0.1700.107 on a Mac Mini running Mavericks).
I want to change some tags that I’ve added to a previously saved item.
I select a saved item and add a new tag to the item. The sidebar with my tag list flashes briefly and, if it’s an existing tag that I added, the tag count increments (if the tag is new it appears with a count of one). When I remove a tag that was already on the item it’s count decrements by one or it disappears if it’s count was already one.
So far so good.
No I select a second saved item and try and modify the tags, removing the existing ones and adding a new one. The sidebar with my tag list doesn’t flash and no tag counts are incremented or decremented.
If I refresh the screen or close the tab and reopen a new tab, I can modify the tags on the second item I selected previously.