Trace Flag 3688 not working on SQL Server 2014
That trace flag isn't documented and shouldn't be used in production environments.

That said, it appears that the trace flag stopped working in SQL Server 2008 R2 and newer.  As it's undocumented Microsoft won't take a support ticket to fix it as you shouldn't have been using an undocumented trace flag.

All undocumented commands and features can be changed at Microsoft's whim.