The only thing what it might be is that you killed the last dragon whyle you arent controlling your main caracter this also counts for level 20 and the 100% court aprovel in orlais so be sure that you play with your maincaracter to get them unlocked
Same problem for me (Xbox One).
Same thing happened to me. Killed all 10 dragons, the game journal shows this but no achievement.
I fear it may have something to do with another bug, the one where Greater Mistral cannot be attacked, as posted about here
Please fix this this, as this is not a minor achievement (they're all important but this one is very important)
