I have just given this a quick run-through on my test site (14532ad425) and the Bulk Append Tags seems to be working okay.
- Go to Latest
- Open topic select and select a sample of topics
- Open gear menu and ‘Append Tags’
- Choose tag(s) and ‘Append’
- All selected topics are now tagged with those tags
Is there something else that I could try that may be different on your site?