Source: I am the maintainer of the typescript-eslint project.
If you update your versions of @typescript-eslint/parser and @typescript-eslint/eslint-plugin to v4.1.0, you will be able to use the latest changes which make @typescript-eslint/no-unused-vars work properly for all cases.
As an aside – using v3.x of the plugin but v4.x of the parser will put you into a really weird state with undefined and unsupported behaviour.
You should make sure you are always using the same version of both packages, as each version is released together.