"Vue.set" doesn't works

[Terence-Cheng]

Version

2.6.11

Reproduction link

https://codesandbox.io/s/prod-http-o0x6q?file=/src/App.vue

Steps to reproduce

If I assign a value to a property that is not defined in advance before using the Vue.set, then this property is not responsive.
f I delete the code of "assigning the value", then it works.

What is expected?

The value I set is not shown in the html.

What is actually happening?

The value I set is shown in the html.

[posva]

Duplicate of #8129