The system detects if the tag is in the title, and adds the first tag that's not already in the title. Since the system detected that "Lua -" was already in the title, it added the next tag, curses. If all the tags have been added to the title, no prefix will be added in the title bar.
I just removed the "Lua -" section from your title and now it shows "lua -" in the title bar.
As a rule of thumb, try to work tags into your title organically - you wouldn't normally prefix a sentence in English with the subject. So, "Do something in Lua while waiting for a blocking curses call" would be fine (and take care of two tags).
