One verse changed the tribe's meaning. Listing the forces that came to David at Hebron, the Chronicler describes the men of Issachar as those who had understanding of the times, to know what Israel ought to do. In context it probably means political judgement, knowing when to move. Later Jewish tradition read the times more literally, as the reckoning of months and festivals, and made Issachar the tribe of the calendar and of the study house.
That reading joined itself to the neighbouring blessing over Zebulun, whose lot is placed at the haven of the sea. From the two, the rabbis built the Zebulun and Issachar arrangement: Zebulun goes out to trade and supports Issachar, who studies, and the two divide the reward. It became a named institution rather than a metaphor, invoked in medieval and modern Jewish philanthropy as the formal basis on which a merchant may fund a scholar and share in the merit of the study.
The Testaments of the Twelve Patriarchs, a later work outside the Hebrew canon, takes a third direction and gives Issachar simplicity as his virtue, a farmer who works the ground and does not look at his neighbour's wife or his neighbour's field.
Three readings of one tribe, then: the ass under its burdens, the scholar of the calendar, and the plain farmer. All three are built out of a handful of verses, and the middle one, which has the least support in the text, is the one that gave the name its long afterlife.