A new article from Sean Anthony: Muḥammad, Menaḥem, and the Paraclete: New Light on Ibn Isḥāq’s (d. 130/767) Arabic Version of John 15:23-16:1,
Anthony argues first that Ibn Isḥāq’s version was translated from Christian Palestinian Aramaic. This part updates Anton Baumstark, "Eine altarabische Evangelienübersetzung aus dem ChristlichPalastinenischen" (1932), now translated in Ibn Warraq's
Christmas in the Koran as "An Old Arabic Gospel Translation from the Christian-Palestinian", pp. 297f. Anthony acknowledges this debt in two footnotes. Anthony also points to Evangeliarium Hierosolymitanum as an exemplar of relevant CPA, and to Climaci Rescriptus as also CPA but not with relevant text
here - in both cases, as Baumstark had done before him.
Anthony brings an unedited fragment of Abu Ja'far Ibn Abi Shayba's Tarikh to triangulate with Ibn Hishām; which means, he can do what Baumstark couldn't, which is to constrain that translation before Ibn Isḥāq’s time.
Anthony is correct that CPA-translations are (still) rare in what's been uncovered from Islamic literature. Ibn Isḥāq himself doesn't use CPA-translation elsewhere; Joseph Witztum, "Ibn Isḥāq and the Pentateuch in Arabic",
Jerusalem studies in Arabic and Islam 40 (2013), 1-72 had argued that Ibn Isḥāq's
Torah quotes derive from an Arabic translation of the Peshitta Old Testament. Which was in classical Syriac with help from Jewish Aramaic, IIRC.