Version

3.1.1

Reproduction link

https://codesandbox.io/s/vigorous-swanson-31rrq

Steps to reproduce

You should open the example and you will see that a component without a class which I added on my component

What is expected?

I am expecting the classes to be applied

What is actually happening?

The component without a class name

This worked in previous versions (2.6.11 and 3.0.8): 2.6.11 -> https://codesandbox.io/s/optimistic-kirch-06cmt

It seems like a new bug after this fix https://github.com/vuejs/vue-next/pull/3742

I'd be suprised if this was "working" in 3.0.8, since this behaviour is an explicitly documented breaking change in 3.0.0:

https://v3.vuejs.org/guide/migration/attrs-includes-class-style.html#migration-strategy

If this "worked" in 3.0.8, that would have been a bug.