What's up with the "politarchs" of Acts 17:6, 8? Did scholars at one point believe Luke had got it wrong?

Yes, so it seems. The unusual word politarch (literally, city official) shows up only in Acts 17, and for some time extrabiblical examples were unknown. Some skeptics doubted the authenticity of Luke's term. However, he has been fully vindicated. Today the term is also evidenced by some 65 inscriptions—almost half from Thessalonica. As usual, Luke's geographical knowledge is impeccable.