VueJS Using Prop Type Validation With NULL and 'undefined' Values?

It's because of required: true

From API docs (more detail)

required: Boolean Defines if the prop is required. In a non-production environment, a console warning will be thrown if this value is truthy and the prop is not passed.

// Basic type check (null and undefined values will pass any type validation)

This applies only when required: true is NOT set. In your code, you are saying that the prop is required and so Vuejs is showing the warning

Related discussion: https://forum.vuejs.org/t/shouldnt-null-be-accepted-as-prop-value-with-any-type/63887