What the bug?
Missing new BEM styling versions of the the responsive visibility classes. Older style still is usable but the new version (reflective of updated documentation) does not seem to be provided. This includes classes such as slds-show--x-small being found while slds-show_x-small is not found.
Found to impact all new(2.3.0) and old (pre 2.3.0) utility responsive visibility classes.
Found in app with <extends="force:slds"> provided version of 2.3.0 as well as version currently rendering at www.lightningdesignsystem.com. This was found in sandbox running summer 17.