Functional component fails depending on the siblings around it

[lykahb]

Version

2.5.16

Reproduction link

https://jsfiddle.net/lykahb/9grbbt4b/

Steps to reproduce

1. Create a functional component that returns several root nodes
2. Use it with is binding, like <div is='my-component'> with no siblings around it
3. Render the template

What is expected?

Renders correctly

What is actually happening?

Renders a string "undefined" with no console errors

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It is interesting that it may render successfully depending on the siblings around it. There are several examples in the link.

[posva]

Works using component instead of div though 🤔