Yes, you are referring to “Rumors of Mechagon” quest, which I did. This quest finishes at Gazlowe, who takes us to spy on Alliance in some Junkyard.
Okay, I found a blue post which mentions one more achievement, Rustbolt Rebellion and I don’t have it … Interestingly, this achievement is not shown as required when clicking Mechagenome race flag in the Embassy.
Now have to be exalted with Rustbolt Resistance to see if it solves the issue where I didn’t get quest to unlock Mechagenome.
Now for other problem where I got Dark Iron Dwarf unlock quest, my alliance character doesn’t show that I have the achievement, however my horde character has required achievement, i.e. Ready for War.
One confusing thing is that “The Mechagonian Threat” achievement is earned on Horde, which is showing when clicking Mechagenome flag but Ready for War isn’t.