Tip by Scottish.7148 ACTUALLY WORKS! Thank you.
TL;DR version
1. Re-start story instance
2. Wait for Rytlock to cross bridge
3. Pick up pages and complete first achievement
4. Kill mobs in cave
5. Rez Breigarth
6. Kill mobs in first clearing
7. Grenade tendrils, complete second achievement
8. Kill Overseer
9. Run back to Breigarth in first clearing and talk to him
10. Rez 4 NPC’s (3 in first clearing, 1 up in Overseer clearing), complete third achievement
The thing was after killing the mordrem in the cave I res Beigarth and then rush up to kill the second pact of mordrem and that caused the bug. What you need to do is to TALK to BEIGARTH after you res him. He will activate the event “Beigarth’s Team Members Found 0/4”
Hope this helps.
:)