Where a labor right arises from a statute but is waived by a CBA, a challenge to the waiver is construed as a challenge to the statute, not to the CBA, making preemption removal inapposite and depriving the federal court of jurisdiction over the question of state law.
Dissent: It's about the CBA.
http://cdn.ca9.uscourts.gov/datastore/opinions/2018/08/31/17-15767.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.