Piracy

You'd have to compare the good and bad files in a hex editor and see how they differ, and if it's really corrupted somehow or if you got the first couple MBs with the rest as random padding to make it look like you're downloading the whole file at super fast speeds. Would work for smaller files or the first couple seconds of a movie or whatever so most users wouldn't notice.

In one way it could be a tactic to get you to pay, but at the same time it gives a bad impression of the service. So I'd be surprised if they intentionally did that. It could also just bug out.

Those sites also fundamentally exist to abuse premium accounts on the big file host sites and resell it for cheap. So the file hosting site may be sort of shadow banning those accounts such that it gives corrupted files which the debrid site then happily pass along back to you, at full premium speed. If I was on the defense site that's absolutely what I would do, otherwise they'd know immediately and switch to a backup account or buy more accounts and make it less obvious. It ruins the end user's confidence in those sites and makes it a headache for the admins to even detect when this happens short of users complaining.

Fun fact, debrid comes from french "débrider", "débrideur" which roughly means unleash/unleasher. Those sites originated from french communities often as WhateverDebrid and I guess it's just stuck once it went international that they're "debrid" sites.