Inconsistent behavior between functional component and normal ones

Version

2.5.16

Reproduction link

https://codesandbox.io/s/9y8kz86624

Steps to reproduce

1. create a component with a slot, and add some fallback content to the slot
2. create a component with render function h => h() and use it as the slot content
3. create a functional component with render function h => h() and use it as the slot content

What is expected?

Fallback content should be rendered.

What is actually happening?

For the normal component, fallback content is cleared and a comment node is rendered. For the functional component, the fallback content is rendered.

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Related: https://github.com/vuejs/vue/issues/7829#issuecomment-373131958

[robcresswell]

Is https://github.com/bootstrap-vue/bootstrap-vue/issues/1680 related to this? We seem to have some odd failures in Bootstrap-Vue that several people have flagged as only occuring in the 2.5.16 and not beforehand.

[yyx990803]

Also: this is not related to #7958 or the regression found in bootstrap-vue.