Hi Mark,
Greetings from Syncfusion support.
We have validated your query and the issue occurred because the dialog element is not removed, we are glad that you found the workaround solution.
We have also considered “Kanban edit dialog element not removed when 'args.cancel' is set to true on 'onDialogOpen' event” as a bug from our end and logged the report for the same and it will be included in the patch release on 1st June 2021.
Regards,
Revanth