The artifacts (debug samples, models, artifacts) deletion is currently performed done from the browser, after the task object is deleted from the backend. So it might take time for all the artifacts to be deleted - please make sure not to close your browser.
Note that only artifacts saved on fileserver are deleted - exteranal artifacts (in the cloud or on a local filesystem) are not deleted.
We are working on moving the deletion to the server side, to avoid these kind of issues. Should be in one of the upcoming versions