Dark Souls II Discussion
Trainer Tools and Resources
Semi-similar thing happened to me when I went through the game the first time using this cheat - I walked up to the Ancient Dragon and talked to him, got the Ashen Mist Heart as a result (so at least I could continue the game), then attacked him with "Easy Kills" turned on and when he died he did not drop his usual items (Giant Soul, Petrified Dragon Bone).
Solution I think will work would be to talk to him first to make sure you get the Heart, deactivate Easy Kills and attack him until you see his health bar, indicating the game has repurposed him as an enemy, then reactivate Easy Kills and take him out.
In your case, the only non-item editor way I can think of to continue is to use a Bonfire Ascetic on the bonfire at the Dragon Shrine entrance to respawn the Ancient Dragon. Don't know if he'll still be hostile if you do that (run up to his location and if there's a fog gate, he's hostile - if not, he's not), so if you want to try my above method of talking to get the Heart (the only necessary item to continue) and then attacking, you might first want to warp over to Brightstone Cove Tseldora and talk to Cromwell to "pardon your Sin," which will de-aggro him.
I haven't actually tried this though so I have no idea if it will work.
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