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.