Yeah, I ran into that too. I reset the mission thinking it was bugged... Not that I went the wrong way. Oops.
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I experienced this the first time I played this mission. I just assumed it was telling me to go into the Hive fort entrance that IT SPAWNS YOU LOOKING AT and worked my way all the way down, following the waypoints, to that dang door. My 'mistake'. I just restarted the mission. Bit of a bummer though.