Troubleshooting this post-function configuration: In case the post-function does not work as expected these are the things to look out for
When you add this post-function to a transition and trigger it, the add-on clears the selected field(s) of the issues linked to the current issue through the specified link type.
Run as current user: The current user will be the author of the change.
Run as add-on user: The add-on user will be the author of the change.
Run as this user: Any user selected in this field will be the author of the change.
If you select any option other than "Run as add-on user", so that the assignment appears to be done by the current user or a specific user, the selected user will need to have the Edit Issues permission.