Hiding in plain sight...From Luther's Works V47 in the preface to "The Jews and Their Lies" is a plausible explanation...
key figures in both the Protestant and the Roman Catholic camps had issued blasts against the Jews. The Strassburg Reformer Martin Bucer published a treatise On the Jews in 1539 which caused him to be regarded by the Jews, for the time being, as their chief antagonist among the Protestants. Compared with Luther’s subsequent proposals, however, Bucer’s program for dealing with the Jews was relatively moderate, though the area of overlap is obvious. No new synagogues were to be built. The Jews were to refrain from “insulting” Christianity, and were to be compelled to attend Christian sermons. They were to abjure whatever the Talmud had added to the Scriptures, to be barred from all business activity, and to be assigned to menial tasks. The Jews, Bucer declared, are implacable foes of the true faith, just like the papists and the Turks.
Luther, M. (1999). Luther’s works, vol. 47: The Christian in Society IV (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds.; Vol. 47, p. 129). Fortress Press.
Apparently the mindset of the Middle Ages was strife with antisemitic sentiment and Luther was awash in that type of thinking.