In certain conditions, the Test Execution filter does not work with unassigned Test Runs

In certain conditions, the Test Execution filter does not work with unassigned Test Runs.

Steps to reproduce:

1) Create a new Test issue.
2) Create a new Test Execution issue and add the previously created Test issue. Make sure that the issue is unassigned.
3) Go to the Test Run page and assign the Test Run to any user.
4) Refresh the Tets Run page.
5) Now, set the same Test Run again to "unassigned"
6) Go to the Test Execution issue and filter for "unassigned" in the "Test Run Assignee" field.
This time you won't be able to find the expected test issue in the table results.