You can click on the tag (twice I think) to see tag view for that tag which would show you all tasks with that tag, grouped by project.
Can you use the Assignee field for this? If so, you can sort (group) by this field.
If you’re on a paid plan, you could save an advanced search report with tasks with that tag only in that project and see just what you want there.
If you’re on a paid plan, you could use Custom Fields instead of Tags and then you could sort (group) by that field and see your tasks grouped together.
Finally, you can use the free-to-most Asana2Go (disclaimer: I’m the creator) to filter as you type to accomplish this:
Larry