When creating new subtask, all tags and subscribers are being copied from parent to it. That is not always desired. Apart from tracking tasks, where most people want to watch tracking task itself (and then possibly subscribe to subtask they are interested in) and the subtask is not tracking anymore, there are also tasks in hierarchy Epic → Story → Task where the former two use #epic or #story tag thus copying it to the subtask isw again undesirable (and same issue with subscribing as for #tracking mentioned earlier applies - somebody wants to watch just the epic itself ie.)
People often forget to remove such tags, not speaking about that they can't know about who to unsubscribe and who to keep.
There should be a way (ie. global setting "maniphest.copy-tags-to-subtaks" & "maniphest.copy-subscribers-to-subtaks") to disable this automatic feature, as it mostly creates more harm than good.