Since the upgrade to Jira 6.1.6, and MWE 3.2.0, the post function "Assign to last role member" does not work anymore (correctly) when selecting "use selected assignee if changed on transition screen". The issue always (!!!) gets "unassigned", even if the assignee was modified in the transition screen.
The post function is the first post function in that transition, no other post functions were added.
The post function seems to work correctly when selecting "Force assignment"
I did a lot of testing. I removed the post function to be sure no other thing is changing the assignee to unassigned. I tested different combinations of setiings of the post function, I can not get it work when selecting that "use selected assignee..." option. The assignee is always set to unassigned