I know two other guilds experienced this. For now make sure you do not pop the DZ for Sev and go in until you are 100% ready to engage. After about 8-10 minutes Sev does his little shadow step/depop/repop trick he does but doesn't repop in the instance like he does open world. You need to engage him in the first 5 minutes. Until the Dev's fix the DZ Sev dz just don't pop it until you are there in full force buffed ready to run in and engage.