[Vuejs]-For what reasons script behavior related to DOM can change in particular browsers?

Solved

In my case there was a method inside the "created" hook of a Vue instance that was calling an AJAX which was allowing the render function v-for to render the content which I was trying to access by JS.

For general cases I concluded that if something works "sometimes" then this "something" has an asynchronous background. (Look for an AJAX)