Trouble with assignments during transition

Description

I have a transition that includes the following Post function:
"Assign the issue to the last user from the Developers role who had this issue assigned before (unless user selects an Assignee in the transition screen) ."
1. The transition screen opens with me as the new assignee. If I do not change it, the Issue is assigned to me, not the developer.
2. If I change the assignee to someone else, the issue is STILL assigned to me, not the developer and not the other person I selected.
If I change the Post to remove the "(unless user selects an Assignee in the transition screen)" part, then it works as expected: It gets assigned to the last developer regardless of whether I change the assignee field or not.
Is this a bug, or a setup problem?

Environment

None

Customer Info

None

Assignee

Unassigned

Reporter

George Carvill

Labels

None

Internal Complexity

Unknown

Internal Value

Unknown

SEN

None

Requires documentation

None

Impact

None

Fix versions

Affects versions

Priority

Major
Configure