What i did:
- i opened all chests (more than 10 pcs) in a single TIV helltides w/o leaving helltide boarder and did not die.
Is there a trick how to get it? 
Must be. There are always bugs EVERY DANG SEASON for the journey. Nothing new. Try again later. 2 seasons ago i found 3 journey things completed that i never did. Thanks Blizz.
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Update: It worked. It showed as finished as soon as i unlocked the Destroyer tier.