Using v-for on a functional component which returns multiple nodes renders "undefined"

[Herteby]

Version

2.5.17

Reproduction link

https://jsfiddle.net/Herteby/r2nb09eg/

Steps to reproduce

As the title says, or just look at the fiddle :)

What is expected?

The slots should be rendered properly.
The combination of v-for and "fragment" should return an Array of VNodes.

What is actually happening?

The slots render as "undefined"
It returns an Array of Arrays of Vnodes.

---

The problem seems to be that the result becomes an Array of Arrays of VNodes. Seems like it ought to be flattened.
For some reason, if you add an extra element as a sibling, it fixes it.

[posva]

Duplicate of #8468