Would you be surprised at all if there was a re-emergence of slavery in the United States?

Not all prisons, because some states don't allow the exception that exists in the 13th Amendment to be implemented in their state prisons. There is still a lot of coercion and inmates are massively underpaid for their work in a way that is slavery in everything except name in those states.

So if a prison has a work program for inmates, assuming it is slavery is a good assumption. But some prisons don't let prisoners work for various reasons, so not all prisons practice slavery.