Hi, @Plainnn
There was a previous Community topic addressing use of this.$auth which I think may be the answer you are looking for. The short explanation is that $auth is only applied to Vue components; used elsewhere, it will return null. Please take a look over the solution in the earlier thread for further details and let us know if that does not resolve the issue for you.
You can find that topic here: The prototype this.$auth in Vue main.js always null