Maybe an influence of a syro-palestinian translation? In middle arabic, this is common (BLAU, A Grammar of Christian Arabic, § 112.1)
Would you maybe give the complete reference of the Juan Andres' quotation? Could be helpful! Thank you!
Juan Andres, a former Muslim who adopted the Christian Faith, quoted al-Qur'ān in Latin characters undoubtedly by memory (his Refutation was published in the 16th century), and changed in his quotations اجمعون into اجمعين which he considered more correct [!] Here the Marciana attests the very condemned hyper-correct Qur'ānic variant!