**Steps to replicate the issue**
* Create a Phabricator task.
* Using the {nav icon=anchor,name=Edit Related Tasks... > icon=level-down,name=Create Subtask} links in that task's sidebar, create a subtask of that task, //without// changing the default "Status" value for the new subtask being created.
**What happens?**
No notification/transaction is sent to the parent task to inform it[s subscribers] of the new blocking subtask that's been created.
If the subscribers list of the new task is pruned prior to its creation (as IIUC would happen automatically with {T239378}, and as [[https://www.mediawiki.org/wiki/Phabricator/Help#Creating_a_task:~:text=Please%20review%20and%20adjust%20the%20project%20tags%20and%20the%20subscribers%20before%20creating%20a%20subtask!|might be currently recommended]]), that task's subscribers therefore won't be aware of the new subtask unless they happen to check the task-graph themselves later on.
**What should have happened instead?**
Compare this behaviour to what happens when you create a subtask following the steps above, but - in the task creation form - set "Status" to //anything other than `Open`// -- when creating a new task with a non-`Open` status, a "<user> created subtask <task>" notification //is// sent to the parent task! (See https://phabricator.wmcloud.org/T193 for examples of this on Test-Phab, and T403982#11185954 for the example I noticed of this happening in production Phab.)
This notification/transaction should presumably therefore also be being sent when a subtask is created with the `Open` status.
**Software version**
[[https://gitlab.wikimedia.org/repos/phabricator/phabricator/-/tree/b9c1b0839aba4e9e305b1192f45166cc6cd74b14|b9c1b0839aba]]
**Other information**
* I'm filing this as a bug report (rather than as a feature request) as it seems like the functionality to do this might actually already exist? But it might just not be happening all the times that it's presumably meant to happen.
* https://gitlab.wikimedia.org/repos/phabricator/phabricator/-/blob/wmf/stable/src/applications/maniphest/xaction/ManiphestTaskUnblockTransaction.php seems potentially related.
* See also https://gitlab.wikimedia.org/repos/phabricator/phabricator/-/commit/57909a705ca832001e58d6bef4af41f6c019bacd