When it comes to auto-completion, texstudio only seems to know the deprecated \bf and friends. The new \bfseries and friends do not appear in the list of command. \bf and friends have been deprecated for some time no, so I'm calling this a bug.
For a list of commands see http://www.forkosh.com/pstex/latexcommands.htm
Here are some sites confirming that \bf and friends are deprecated: