Why is v-model not type safe?

[RobbeCl]

When the props and emit type do not match, the Vue Typescript checker does not complain, why is that? When working with :model-value and @update it becomes type safe.

For example: prop=string and emit=string|null

https://play.vuejs.org/#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

Vue-tsc (version 2.0.6) output:

Test1.vue:10:58 - error TS2322: Type 'string | null' is not assignable to type 'string'.
  Type 'null' is not assignable to type 'string'.

It only hunts at the second invocation

[vieruuuu]

v-model still not works, only version 2.0.17 is working, anything newer is broken.