“Issachar is a strong ass, couching down between the sheepfolds,
And he saw a resting place that it was good, and the land that it was pleasant,
And he bowed his shoulder to bear, and became a slave under forced labour.”
Jacob recognises in Issachar (‘man of wages') someone who enjoys his pleasures and lacks initiative. He would always be a servant to others rather than taking the mastery. He would always prefer to be paid rather than being an entrepreneur.
“A strong ass.” The ass was a beast of burden, and Issachar is pictured as being ready to receive extra burdens as he does his work among the sheep. It may be that in Egypt he had fixed his eyes on its pleasures and in order to enjoy them had become committed to a certain level of forced labour in order to subsidise a pleasurable lifestyle.
There is no evidence that the tribe of Issachar became specifically a slave nation, but its territory which was in the vicinity of, at times, strong Canaanite cities whose fortunes varied (Judges 1:27-28) might suggest that it would itself at times be subject to strong outside pressures and never fully establish itself. Consider its non-mention in Judges 1, possibly there being included in Manasseh. However it was lively enough in helping Deborah (Judges 5:15), and there is no real reason for seeing it as especially enslaved.