FOTPT only applies in instances where the cops obtained the content illegally. If some vigilante or bad actor obtains evidence of a crime without coordination or involvement from the government, then the government obtains this illegally-obtained information through legal means, it's still fair game for prosecution.

For example, if I break into your home and steal your safe, which contains evidence of fraud you're committing, then the cops catch me and seize the evidence of my crime, thus observing evidence of your crimes, the evidence they took from my case can be used against you.

In fact, if I'm talking to my cop friend and he tells me about the case against you and that they think there's evidence of your criminal activity in your safe, but they haven't obtained probable cause, I could break in, take the safe, and provide it to my friend and it still be admissible in court. Your defense would have to make a convincing argument that my cop friend asked or coordinated with me to commit the crime of breaking into your house/safe.