To my knowledge, the custom kanban functionality is no longer being developed or supported. It is one of the reasons I recommended that you not use it at all. Although, in the Release Notes for 10.55 (dated September 2019) they do mention a slight update.
You can always open a defect with Jira Align support to see if they are actively supporting custom kanban functionality and if they will fix the cumulative flow metrics.
I recommend you focus on the backlog kanban functionality and the value stream/process step functionality.
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