I am reading Marcuse's "Reason and Revolution", and I think it might be a good base for reflection on the similarities between post-positivist social science and 'post-truth' and anti-system politics.
A great deal of philosophers have said far more insightful things than Marcuse on the concept of truth. But I wanted to share with you an inconvenient truth, so to speak: the Frankfurt School and anti-systemic voters in Europe and the United States share many, many similarities (actually just one).
Marcuse uses Hegel to distinguish fact from truth. For him, truth corresponds to an object's and a subject's potential, especially potential for something normatively better, whereas facts represent the status quo. If we stick to social issues (and put the natural sciences aside), this definition of truth seems reasonable (although Marcuse never explains the source of his value judgements). Marcuse calls the recognition of contradictions between facts and truths 'negative thinking', which can also be called critical thinking.
In the book Reason and Revolution and elsewhere, Marcuse uses economic examples (inequality, monopolies, arms spending) to demonstrate what he means by contradictions. Again and again, he deplores complacency and calls for reason to instigate a revolution against the (liberal) status quo. When this book is read in today's context, one can only come to the conclusion that anti-system voters have made a reasoned diagnostic similar to Marcuse's and have voted to change the system -- only many of them are reactionary and not Marxist, i.e. they differ in their revolutionary remedies.
It might be possible to argue that Horkheimer and others from the School were more attuned (than Marcuse) to the issue of how demagogues and nationalists can use 'reason' to their advantage. Marcuse seems to think the world will always be like 1950s USA, other Frankfurt scholars were less pessimistic/optimistic.