I boiled it down to the following:
Visiting the URL https://www.graph.cool/forum/clicks/track?url=https%3A%2F%2Fstackoverflow.com%2Fquestions%2F45253671%2Fadd-an-array-of-objects-to-a-mutation-in-apollo-react&post_id=758&topic_id=365 in a normal tab doesn’t work for me.
Visiting it in an incognito tab works.