Version

3.0.5

Reproduction link

https://codesandbox.io/s/cranky-einstein-pfvg9

Steps to reproduce

SFC:

<script>
const myMixin = {
  created() {
    console.log('hello from mixin!')
  }
}

const myExtends = {
  mixins: [myMixin],
  created() {
    console.log('hello from extend!')
  }
}

export default {
  extends: myExtends
}
</script>

What is expected?

Console print out:

hello from mixin!
hello from extends!

What is actually happening?

Console print out:

hello from extends!

Vue2. x is correct.