Can anyone prove Job was not a son of Issachar?
"The sons of Issachar were Tola, Puvah, Job, and Shimron." (Genesis 46:13)
The biggest problem is that no sons were listed for Job when they all went down into Egypt, despite all the other male children being listed, including Judah's dead ones.
Now, it could be that none had been born yet, but that would then put the land of Uz in Egypt. I'm not sure how much sense that makes.
It could also be that it was between families. I.e., that it was after the 10 children had died, but before the 10 replacement children were born, but then why weren't the dead ones mentioned like Judah's dead sons.
I think it makes more sense that it was from a former time, maybe back closer to where Abraham came from:
[Gen 22:21] 21 The oldest was named Uz, the next oldest was Buz, followed by Kemuel (the ancestor of the Arameans),
Since Job was from the land of Uz (an Uzite, probably), and one of his friends was a Buzite, they were likely offspring from Nahor, and therefore Job was likely a cousin of Rebekah. That could explain also how the story came to the Israelites--through communication between Rebekah and her family, or possibly Jacob learned of the story when he went there and married Leah and Rachel.