There’s a max number of App Access Tokens that you can have, after which when you create more the oldest active one will become invalidated.
So yes, there is a chance that one of your threads could have an invalid token if you have many other threads also generating App Access Tokens. This problem will continue to grow if your apps generate a new token when they detect that theirs is invalid, as that could then just kill the token another thread is using, which again will create a new token, and kill the oldest etc…