If a post-function is added to any workflow to re-assign the default assignee at the initial create transition, the function will silently fail and the issue will remain assigned to the project default assignee or the component default assignee.

At other transitions the 'update issue field' does perform as expected.

Other related post-functions at the 'issue create' transition do work as expected, such as the 'Assign to Reporter' post-function. Testing against other combinations has not been exhaustive.