Accessing a Thread variable in Rails Controller Test fails

I have a Ruby on Rails 5 application where I make use of a Thread variable to access the user initiating an action for my own proprietary logging. All is working fine until it comes to my automated tests.

I have a Module in lib/current.rb:

module Current
  thread_mattr_accessor :user
end

In my app/controllers/application_controller.rb, I set the current user like this:

around_action :set_current_user

[...]

def set_current_user
  Current.user = current_user
  yield
ensure
  Current.user = nil
end

With that, I am able to log the currently logged in user when creating/updating/deleting an entity in my application, e.g. in app/models/entity.rb:

after_create :log_create
before_update :log_update
before_destroy :log_destroy, prepend: true

[...]

def log_destroy
  ApplicationController.helpers.create_change_log([...], Current.user.id)
end

As stated above, that is working fine until I try to run my automated tests:

entity_instance = Entity.find([...])
entity_instance.destroy

This leads to the following error message:

Minitest::UnexpectedError: NoMethodError: undefined method `id' for nil:NilClass

I have assured that the error points to the non-existence of Current.user. Does anybody have an idea why this could be the case?