Where did you hear this? I watch channels like Lack Luster and Audit the Audit and they never mentioned this, but instead stress the importance of recording for yourself.
it's a long held concept in trials.
If someone like Bernie Madoff has a team of 10 people show up at his offices and start shredding documents after he learns he's about to be arrested, the jury is going to be instructed that, even though the alleged evidence was destroyed, it can be inferred through the defendant's actions that the shredded documents were incriminating without any evidence to prove such.
It's "common sense" that someone who destroys evidence is doing so to cover their ass. If the evidence were favorable to their case, they would have every reason to preserve it.
Now that there have been documented cases of departments "losing" footage from body cams, and in some departments on a routine basis, it's obvious the coincidence of that with a formal complaint infers a deliberate cover-up, and as such can be interpreted by a jury to be evidence against whoever is being accused (might not even be the wearer, but someone the wearer's camera recorded).
This paper explores the remedies courts can and should apply when entire videos, or portions of recordings, are missing.
In summary:
Where video is missing altogether, an
inference that the video could contain information favorable to the
defense should be drawn. This contrasts with spoliation doctrine,
which often requires a showing of bad faith before imposing the
adverse inference that a party destroyed or failed to preserve
evidence because it was favorable to the other party.
https://digitalcommons.law.uw.edu/faculty-articles/311/Having such rules will motivate leadership in departments to protect and manage camera footage, or else risk a case falling apart because the absence of that evidence will be held against the prosecution.
Of course, it's up to the jury to make that connection with the facts. Not many juries will be sympathetic to a department that "loses" evidence critical to proving a case against them.
I can't name a specific case where that's happened off the top of my head re: body cam footage, but if I have time later, i can search -- or wait for you to find some.
