Per Heroku’s documentation “When running multiple dynos, apps are distributed across several nodes by the dyno manager. Access to your app always goes through the routers. As a result, dynos don’t have static IP addresses.”
Therefore it’s difficult for me to know which IPs are affected. I suggest you reach out to Heroku directly. You can reference my Heroku support ticket #94332. My contacts there have been Chris and Brett.
Please let me know if there’s anything else I can do to facilitate communication between you guys. Is there someone at Twitter I can tell them to reach out to? If you make this discussion public, I can tell them to post here.