Why does a complication still show an old timer?

Watch face complications are not live views that refresh every second. watchOS controls when complications are reloaded and redrawn. For that reason, a timer that has ended or was deleted may remain visible for a while.

Open LongTimer on the iPhone and Apple Watch to encourage a refresh. If the issue continues for a long time, restart the Watch and iPhone and check that you are using the latest LongTimer version.