What "loophole" is left in the amendment, that could potentially allow for some kind of involuntary servitude to continue?

The 13th amendment states that slavery is allowed if you are a prisoner

Explanation:

Some Big Corporations rely strictly on prison labor this can be exploited by arresting citizens that have done nothing wrong.

The 13th Amendment, ratified in 1865, says: "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction." Scholars, activists and prisoners have linked that exception clause to the rise of a prison system that incarcerates black people at more than five times the rate of white people, and profits off of their unpaid or underpaid labor.