NoBru,
Are you referring to the status of the task on the actual task tab, or a report? It can really depend on a few things, primarily the type of task (automatic install vs. manual), whether or not the task is active, and the software event purging policy you have in place. The purging policy is under the configuration tab -> solutions -> software management -> software delivery -> windows -> software delivery purging policy. By default it is 7 days.
So, as an example, if you have an automatic task, enabled and running, after 7 days, the status will be wiped out. So it's not really a bug, but it is a very good question on how all the pieces interact.
Cheers,
Jim