What happens when a co-founder leaves the company post-investment before having fully vested? Does the equity usually return to the other co-founders or return

What happens when a co-founder leaves the company post-investment before having fully vested? Does the equity usually return to the other co-founders or return to the company, effectively giving all shareholders (n.b. including investors) a pro rata increase in ownership? If it's the latter, one must question an investor's incentives in giving that advice, i.e. are they just encouraging a structure that has the most probability of them ending up with more ownership than they paid for? It's actually difficult to filter for bias when your counterpart has diametrically opposed incentives.