First, your solution does not apply to the problem at hand. The requirement is to identify the Tests that are in a Run (and then maybe also a Plan) that are not assigned.
Second, your technique ONLY works at the initial creation of the Plan. This is something I pointed out in my original thread on this.
Once the Plan is created a user can go back and edit the assignments AT THE Plan Level all they want but they will have NO effect. The assignments will stay the same as at Plan creation time.
Once a Plan is created the only way to change assignments is to open the actual Run. And that puts us back at the initial issue of this question.
(I think this behavior is a bug in TR. If I cannot change assignments thru the edit of the Plan it should not allow me to. If these settings, after Plan creation, will only affect new tests cases that are added to the Plan some time later than that should be outlined in the GUI. This last point is what I believe I was told was the use of these settings AFTER a Plan is created.)
My current work-around, if none of the Tests have been executed yet, is to re-run the Plan and set the assignments then which is at creation time of the new Plan. Then I delete the old one.
Lastly there are other cases where I need to find unassigned test cases and not blindly set their assignment.
One example I have right now is somewhere in my two open Milestones with a total of 36 Runs, and ~2000 Tests are 10 Tests that have been marked Passed but are unassigned.
I need to find those and assign them to the actual person who ran them. Neither thru Plan edit or Reports or Todo can I ID what those 10 Tests are.
(I know they are in a Passed state from the TODO page when I filter on only Passed do the Unassigned show up.)
My TR version: TestRail v126.96.36.19913