I walk up talk...end of convo...sound and a flash?

You probably have some persistent buff or condition, and the game is reapplying it after coming out of the conversation scene.

Technically it was applied the whole time, but I've seen some effects (e.g., weapon effects) that disappear in conversations in which my character is rendered without a weapon, and then reapply after the conversation. I assume this is because the game unequips the weapon to render the character without one, and then re-equips it at the end of the scene, but that's just a guess.