It's working fine here and on 4 Xenforo 2.1 forums I work with.
If you use Editor & BB Code Manager | XenForo community, that was creating issues with several editor buttons in Xenforo 2.1.1 but that was updated yesterday and the latest release seems to have corrected all known issues.
Otherwise, if you are using a custom style
first check to see if you still have the problem with the default Xenforo style, and assuming the answer is no
check with whoever created the custom, style - there are a number of changes in 2.1.0 and 2.1.1 that need updates.