Flaky tests with redis

I have a unit test for caching an API key in redis (in a rails app).

redis is set to a global variable ($redis) with the connection_pool gem. So I do $redis.with do |redis| to actually get the redis connection.

The issue is, my test is incredibly flaky. It has two assertions: one to test getting the expected value out and another to test that value is cached (for 10 minutes).

jwt = JWT.encode payload, private_key, "RS256"
assert_equal jwt, Foo.app_token
travel 5.minutes
assert_equal jwt, Foo.app_token

I can't work out why the test is so flaky. It fails maybe every 3-5 runs. Any help would be greatly appreciated. I've never really used redis inside a connection pool so may well be doing something basic wrong.

The implementation of the method:

def app_token
  find_app_token || create_app_token
end

private

def find_app_token
  $redis.with { |redis| redis.get APP_KEY }
end

def create_app_token
  payload = # some payload
  JWT.encode(payload, private_key, "RS256").tap do |jwt|
     $redis.with do |redis|
       redis.set APP_KEY, jwt
       redis.expireat APP_KEY, payload[:exp]
     end
  end
end