If a user is automatically assigned when an issue transitions to a new status, something is doing that; it is probably an automation rule or workflow action. Please check for use of those and remove the one(s) you want, or change them to clear the assignee instead.
That is interesting, and I am not familiar with that default setting for a project.
What type of project is it: team-managed (next-gen) or company-managed (classic)?
Would you please check to see if you have any automation rules set up? You may find them from the board by selecting the lightning bolt icon > manage automation rules. If you see no rules there, it is a workflow rule which is doing the assignment.
When looking at a workflow, you may select one of the lines connecting the status values you are interested in updating. That should show if there is a rule on the workflow, which you may then edit or delete. I believe for classic projects you will need to publish the workflow after that.
Please look here for more details on managing workflows:
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