TL;DR - Setting implies it is filtering on "resolution" or "resolutiondate" fields, when it is actually filtering on "updated" or "updatedDate" fields.
Long Version - With a Kanban board using this filter, normally there's no problems, and the feature appears to work as expected (as long as issues follow only the happy path from creation to resolution). Users see issues completed more than 1 or 2 weeks ago disappear from the Done column on the board, and everything is nice and tidy.
Then, your team, organization, or company makes a change of some type to how Jira is used, and a set of issues are bulk updated to meet the new expectation. Suddenly, the Kanban board is flooded with issues closed weeks, months, or even years ago. Developers are having trouble finding their next ticket, project managers are panicking due to the appearance that many items were reopened for some reason, and everything is terrible, especially because there's no way to correct the issue, other than just telling everyone to suffer through it until the week passes, and the issues disappear again (see screenshots included below).
Kanban Board Filter Settings (sensitive information redacted)
Kanban Board with old issues reappearing after bulk edit (sensitive information redacted)
While the phrasing of the setting is confusing and misleading, especially combined with the help text of "Speed up your board by hiding issues you're no longer working on." below it, the real issue is that the standard expectation is that the phrase "completed older than..." implies that the feature is looking at the issue's completion/resolution date, not the date when the issue was last updated.
While the easiest solution to this confusion would be to change the label and help text for the feature (i.e. "Hide Closed Issues Updated Longer Ago Than..."), the desired behavior, hiding issues with a no-longer-important resolutiondate, is still not met, and any subsequent bulk updates will cause the reappearance of old issues, and reintroduction of confusion to the development team.