There's basically no value in having tests against third-party code anyway, because all the test is going to do is tell you that they broke their interface. And by then, production is already broken.
I agree this is often the case but disagree it is always; “testing” against the api can be a canary for your new usage of their third party api not working the way you think it does.
I don’t know, I think having your tests assume that the 3rd party data looks a certain way is helpful. If that ever breaks in prod, then something needs to change, and your tests can change if the interface changes.