Using addon Create on the transition to create subtask as a post function in one of workflows transition.
Unfortunately, some the subtasks are not assigned correctly. It will get assigned as unassigned.
Steps to reproduce:
#1. Have created the user (testemail).
#2. Added the create sub-task post function with specific assignee as testemail in one of the transition.
#3. I transition the issue, it got created sub-task with assignee as testemail user.
#4. I have changed user name with testemail_old and made inactive.
#5. I have created the another test user with same name testemail.
#6. I transition the same issue again, it got created the sub-task with assignee as un-assigneed. (Issue is reproduced)
For more information, please check the linked support issue below.