I have a similar problem. When I add the task from outside the project and link it to it, after I save and reload the page, I find that the link has been removed.
when you add a task from a task doctype and link it to an existing project, the number of tasks within the project increases by one, but it does not appear within the task table within the project.
If you try to modify the project page, it will unlink the project from the newly added task.
In addition, any change to any task within the same task is not reflected on the tasks within the project, for example change the start date or end date or task name.
But the problem we all have, tasks disappear from one day to another, is not fixed. We need something in v11 that makes it possible to use projects until v12.