it's not a mistrial. it's called a brady violation. during a trial, brady violations are grounds for what's called a "judgment of acquittal", meaning the case doesn't even go to the jury, the judge declares the defendant innocent.

of course, the judge is not going to give a JOA.

https://www.pagepate.com/what-is-a-brady-violation/

https://theappeal.org/the-epidemic-of-brady-violations-explained-94a38ad3c800/

brady violations are on the rise because mass surveillance and phones capturing everything nowadays mean there's more physical evidence than ever. where cops could previously lie, or prosecutors could insinuate to run up conviction rates, they can't anymore. so the only way to eek out convictions sometimes is for them to withhold evidence.