The Kanban boards work a little differently depending on whether they are boards for team-managed projects, business project boards (which aren't really Kanban boards) or proper Jira Software boards.
Team-managed boards and business project boards have a simple, fixed rule that they drop cards that have not been amended for a fortnight (I'm not actually sure what the exact rules are for dropping the issues off, but it is a fixed rule)
Assuming you are looking at proper Jira Software Kanban boards though, then there's a config setting you should understand, and alter. If you go to the board configuration and look at the general settings, you will see the board filter (the query it runs to find all the issues it should display) and then beneath that, a "sub-filter" - that tells you what issues the board is going to display in the "done" column. The default is "don't show issues that have a fix version that has been flagged as released", but you can change it to whatever you want.
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