Don’t know what you are doing different, but mirroring a mirror column is just not supported. You can have two board showing a mirror to the same column on a third board however.
Hi @MaxLogic_Lisa , how are you? I understand you are trying to achieve that certain process flow using mirror column. Have you tried using the same item live on different boards instead? There is a newly launched app called Same Item Multiple Boards by Formula Lookout, let me know how it goes