Seems you're right. The act of leaking Flynn's identity is seemingly an abuse of his civil liberties, but the wiretap itself was likely legal.
It seems rather likely that these conversations were obtained pursuant to an order of the Foreign Intelligence Surveillance Court (as would be required for the monitoring of any person inside the United States, including foreign ambassadors). Publicly disclosing or disseminating such information for purposes other than foreign intelligence almost certainly violated that order, and possibly Flynn’s constitutional rights.
FISA orders and minimization procedures, approved by the FISC, are intended to protect the Fourth Amendment rights of U.S. persons—like Flynn—who are in contact with valid intelligence targets. Minimization procedures are designed to protect incidentally collected information to, from or about U.S. persons. The Office of the Director of National Intelligence has released the NSA’s standard minimization procedures under Title I of FISA.