In Nathaniel Lardner's preface to his volume Jewish and Heathen Testimonies to the Truth of the Christian Religion (Volume 2, published 1765), he writes the following comment about Eusebius and the Testimonium Flavianum.
pages x-xi (https://www.google.com/books/edition/A_ ... frontcover)Indeed, it is not Josephus, but Eusebius, or some other Christian about his time, who composed this paragraph.
Lardner goes on to even argue how Eusebius inaccurately references Josephus on other occasions, that much of the language used in the passage seems to not be Josephan in style. He outright argues that the language is Christian and leaves us to suspect Eusebius in another place too:
Page xi. So we even have a small linguistic argument there as well.Where our Lord is said to be a worker of wonderful works. [quotes the Greek] Which way of speaking is so agreeable to Eusebius, and has such similitude with his stile [ye olden variant of "style"], that I am disposed to put down some instances from him. Which must be of use to satisfy us, that the stile of this paragraph is very Christian, if it not be the composition of Eusebius himself, as Tanaquil Faber suspected.
This is even more helpful. So time to go digging. Apparently Tanaquil Faber (1615-1672) was a French classics scholar (for the day) and apparently wrote Flavi Josephi de Jesu Dom. testimonium suppositum esse (1655), which I assume is what Lardner here references. In which case, doubts that Josephus wrote the passage and that Eusebius was the one who constructed it go back to the 17th century.