Hi Bruce
Your workflow design looks very complex. I didnt understand why you are assigning both rule no 90000007 and also the Org units and positions at task level. Ideally you should code all your approver logic in the rule. Each task can use same rule and you can set a flag in rule binding to distinguish diffeent tasks.
The workflow task is marked as general task and when the agent determination is failing, WF runtime determines all users as agents(including the previous approver as reported by you). This is working as per your design.
If your Rule is based on function module then try to run it in SE37 with same imput values as WF container. If rule is responsibility rule based then go transaction OOCU_RESP to simulate rule.
Regards
Sandy