I am in charge of GDPR implementation at my work.
What you did is not a GDPR violation. Security agent (not Polizei) forcibly removing from you is an assault. I seriously suggest contacting a lawyer.
Whilst I'm not disputing your story, I'm surprised LH shared HR decisions with you (which would be in contravention of GDPR).
LH should absolutely not share an HR decision with a customer. But doing it is absolutely not a contravention of GDPR.