In the Vita Pompeii Plutarch tells us that the Cilician pirates, originally equipped by Mithradates VI of Pontus, as we learn from Appian 63 and 92, worshipped “Mithra”.
They were accustomed to offer strange sacrifices on Olympus and to observe certain secret rites, of which that of Mithra is maintained to the present day by those by whom it was first established.
Like most people I have supposed that the reference here was to Mithras of the Legions, rather than Persian Mihr / Mithra. The last phrase “the present day” certainly suggests Mithras; or that Plutarch thought so, although he doesn’t say who it is that “first established” the rites of Mithra. Perhaps he does mean Cilicians?
But worship on a mountain is not something that we associate with Roman Mithras, but rather with Zoroastrianism. Roman Mithras was worshipped in a cave, while Zoroastrians, as I understand it, favoured high places. The temple at Nemrud Dag, in Commagene, which certainly involves Persian Mithra, is on a hill.
Similarly we are dealing with a bunch of people recruited by Mithridates of Pontus, a king of a semi-Persian kingdom bearing a Persian name which I understand is Mihrdad or Mehrdad, and is still used in modern Iran as a personal name. The rulers of Commagene also used the name Mithridates, as did rulers of Parthia and Armenia.
Much about this reference in Plutarch makes sense as a reference to Persian Mithra. The last part of the statement, however, has to refer to Plutarch’s own time, and suggests that he has heard that Roman Mithras is Persian in origin.
While I get that today we realize Cumont’s theory about Roman Mithraism having originated with the Zoroastrian Mithra is wrong and doesn’t jive the the available evidence, I think sometimes we have a tendency to go to the opposite extreme and set these uncrossable iron gates between these two deities, setting up an either/or dichotomy whenever a reference to a “Mithra” is mentioned by an ancient source. While it great for academic advancement to compartmentalize and specialize for more efficient and productive research, we must never forget that the cultures of the Graeco-Roman world were highly syncretic and all too often conflated various deities and played mix-n-match with them. Mithra being no exception, as he was often conflated with Apollo, Helios, Sol, Attis, and even Sabazios, if I recall correctly. Sometimes such conflations caused writers to refer to various syncretic gods interchangably, such as how Plutarch and Herodotus refer to Isis & Demeter or Osiris & Dionysus interchangably. So even acknowledging the distinction between the Zoroastrian Mithra & the Roman Mithras, they were clearly often conflated by ancient writers, sometimes referring to the Roman Mithras as a Persian god, as I think Tertullian did If I’m not mistaken, etc. So I guess my point is, it may not always be the case that a writer is referring to one or the other, but has instead fallen “victim” to the syncretism of the day.
You’re quite right, of course, and it is undoubtedly a mistake for us to be dogmatic about matters on which the Romans of this period were rather casual.