After <https://trac.webkit.org/r204461>, com.macromedia.Flash Player ESR.plugin.sb is installed directly under WebKit.framework/Resources. Other plug-in sandbox profiles are installed in WebKit.framework/Resources/PlugInSandboxProfiles. As far as I can tell from looking at the code in PluginSandboxProfile.mm, only the latter path gets searched for profiles.
See also bug 160753.
Created attachment 309382 [details]
Comment on attachment 309382 [details]
Clearing flags on attachment: 309382
Committed r216445: <http://trac.webkit.org/changeset/216445>
All reviewed patches have been landed. Closing bug.