Fun theory Vision's conciseness is still in the Mind Stone. When Thanos is defeated, the stones are either all scattered or just the Time Stone (which promptly buggers off) and Mind aren't destroyed. It's flung across the universe, where The Sovereign finds it and Aisha adds it to Adam. Adam awakens, having Vision's morals, but no memories and a sense of danger from when he died. He cristens himself Adam the Warlock. Eventually meets with Wanda in Avengers 5 (where they fight Kang or something) and he remembers.
Also it was in character for him to do that.