Learn Finnish: Hän on vetäytyväinen, mikä olisi ongelma. - He is withdrawn, which would be a problem.

It comes from the verb vetäytyä (to withdraw, to pull back). When you add the -vainen or -väinen suffix to a verb stem, it creates an adjective describing a strong tendency or personality trait. So here, it means someone who has a natural habit of withdrawing, translating as "withdrawn" or "introverted."

Could I just use the active present participle vetäytyvä instead?

You could, but there is a subtle difference in nuance. Vetäytyvä (withdrawing) can sometimes just describe an ongoing action or temporary state. Using the -väinen suffix explicitly turns it into an ingrained, permanent character trait.

What is the root verb behind vetäytyä, and how does the meaning shift?

The base verb is vetää (to pull). By adding the reflexive suffix -ytyä, it becomes vetäytyä, which literally means "to pull oneself" (usually away or back), giving us the meaning "to withdraw" or "to retreat."

Why is the relative pronoun mikä used here instead of joka?

This is a vital rule to remember: joka refers to the specific noun right before it, while mikä refers to an entire preceding sentence or clause. Here, the problem isn't the person himself, but the entire situation of him being withdrawn. Therefore, you must use mikä.