Not doing so would mean that tags would be overidden by any tag events sent by upstream. Also only send a tag event directly if upstream never sent one. By default use GST_TAG_MERGE_REPLACE to override tags that exist in both the upstream event and this element with the ones from this element, but provide a new "merge-mode" property to adjust the behaviour. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7145>