While investigating the cause of vuejs/vue-class-component#65 related to vue-hot-reload not working with vue-class-component, @ktsn found a bug in vue core which he noted here:
vuejs/vue-class-component#65 (comment)

He says

I find there is a bug in core library that accidentally drop late-injected options of constructors. That means, if we use vue-hot-reload-api or vue-loader, they inject some options into component options object after creating component constructors, then the component is instantiated by using constructor with $createElement and the injected options are dropped in resolveComponentOptions function.

And that it is also causing vuejs/vue-class-component#66

Here is the minimal reproduction of this bug by @ktsn
https://jsfiddle.net/vvxLyLvq/2/

I don't know much about it, I'm just creating the issue here for @ktsn

In summary this issue is causing vue-class-component hot-reload to not work and also makes using vue-class-component with CSS Modules impossible.

Thank you!