My money is on Strabo. It is difficult to explain away
1) that Strabo in this passage is referring to a campaign on behalf of Mithridates against the barbarians who lived beyond the isthmus (of Perekop) as far as the Borysthenes (Dniepr) and the Adriatic
*****GK: The Adriatic must be a mistake. This makes no sense at all in the context of 110-108 BCE. Tyras does, whence my suggested emendation. I would surmise that some copyist erroneously substituted "Adrias". There is a similarity in Strabo's account of the fall of Bactria (acc. to some scholars and I agree with them) where he repeats "Asii" as "Pasiani". The correct reading is in Justin.****
which took place when Khersonesos appealed to Mithridates for help against the (same?) barbarians, ie in 110-108 BCE.
*****GK: There is a brief lacuna in the Diophantes inscription after he recaptures Neapolis from the Scythians in 108. But no intimation of any further campaigns beyond the isthmus, just an expedition to solidify the position of Mithradates in Bosporus. And then the glorious inscription.*****
This was preparatory to a campaign against the Romans.
****GK: Which fits in quite well with the events of 88 as described in Appian. And with the Plutarch quote about 91-88. *****