There are other posts related to this, but let's drill down on what's wrong with the UX here:

When you try to filter by assignee and you do not have the subtasks view set to 'Separate' the "Assignees" filter box says (incorrectly): "No tasks have been assigned yet. Set assignees on tasks to get started."

This is wrong, and it should actually say the real reason it is not showing anything.

But even this is suspect, because why not just have it work as the user expects and establish the filter.

It is especially wrong looking if you have Subtasks set to "Expanded" because you can actually see tons of Assigned folks in the list yet it still claims "NO tasks have been assigned yet." Clearly wrong and very confusing. And there's no reason I can see to not just allow to filter such a list as the user wants (by whatever list of assignees he/she picks). See attached image.

Having the "Assignee" filter be depending on another control (which is located in an entirely different part of the UI"... with (or without) proper feedback is just really confusing and bad UI/UX design.

Can we get this a serious look see?