Click on "assignee", or use a post-function that sets it to the reporter on a transition.
JIRA doesn't naturally set the assignee when changing issues though, so when you say "instead of default assignee", you are referring to something your admins have set up for you, and that's what needs to be changed.
It sounds like there is a post function doing it. As Nic indicated, you'll have to get the JIRA admin to change or delete the post function. That isn't out of the box behaviour so someone must have thought it was a good idea sometime in the past.
Teams break work down in order to help simplify complex tasks. This is often done iteratively, with tasks being broken down into smaller tasks and so on until the work is accurately captured in well-...
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