Had tooth extraction/bone graft 3 months ago. Have had bitter taste in mouth since. Dr says area is healed/no infection. Why still having bitter taste?

Could be other cause. The bitter taste could be related to other causes such as sinus issues and related post nasal drip or stuff on the tongue. Would suggest using a saline nasal spray to help break up any post naal drip and brush your tongue to eliminate any trapped food particles. if you dont see improvement in a week recommend followup with an ENT to rule out sinus issues.