Bug: Props in child component defined in mixin are missing with `shallowMount` on @vue/compat

Describe the bug
With shallowMount on Vue 3 + @vue/compat, properties of a child component are undefined when using them through a mixin.

If the property is moved from the mixin directly to the component, then the property is defined.

Also if the shallowMount is switched to mount, the property is defined.

To Reproduce

I created a demo repo where this issue can be reproduced, with tags for various working and not working states. It includes the following important files:

• Parent Component: App.vue
• Child Component: HelloWorld.vue
• Mixin: HelloWorldMixin.js
• Test: App.test.js

git clone https://github.com/cody-collins/vue-compat-mixin-demo.git
cd vue-compat-mixin-demo
npm install
npm test

And the following tags:

• Working: Vue 2 + VTU 1
• Broken: Vue 3 + VTU 2
• Working: Vue 3 + VTU 2 + mount instead of shallowMount
• Working: Vue 3 + VTU 2 + props moved to component

Expected behavior
The second assertion should pass in the broken case