Why does WF 3.5 Resume() immediately Suspend again?

Question

Tearing my hair out. The code for my Resume is pretty simple:

var instance = _runtime.GetWorkflow(instanceID);    
instance.Resume();

But the Resume method does absolute nothing, it doesn't even trigger a SQL hit when I SQL Trace. My logging extract is:

System.Workflow.Runtime.Hosting Information: 0 : TimerEventSubscriptionQueue: adb14f31-274e-4dd9-8e8c-b9bad6501a74 Resume

System.Workflow.Runtime Information: 0 : pending work hc 19959204 added workItem hc 34972553

System.Workflow.Runtime.Hosting Information: 0 : WorkflowRuntime:ScheduleLoaded event raised for instance Id adb14f31-274e-4dd9-8e8c-b9bad6501a74

System.Workflow.Runtime Information: 0 : Workflow Runtime: WorkflowExecutor: Got a suspend request for instance adb14f31-274e-4dd9-8e8c-b9bad6501a74

System.Workflow.Runtime Information: 0 : pending work hc 19959204 added workItem hc 31431550

System.Workflow.Runtime Information: 0 : Workflow Runtime: WorkflowExecutor: Suspending instance adb14f31-274e-4dd9-8e8c-b9bad6501a74

System.Workflow.Runtime.Hosting Information: 0 : WorkflowRuntime:ScheduleSuspension event raised for instance Id adb14f31-274e-4dd9-8e8c-b9bad6501a74

System.Workflow.Runtime Information: 0 : Workflow Runtime: WorkflowExecutor: Got an unload request for instance adb14f31-274e-4dd9-8e8c-b9bad6501a74

System.Workflow.Runtime Information: 0 : adb14f31-274e-4dd9-8e8c-b9bad6501a74: Calling PerformUnloading(false) on instance adb14f31-274e-4dd9-8e8c-b9bad6501a74 hc 48165764

System.Workflow.Runtime Information: 0 : Workflow Runtime: WorkflowExecutor: Unloading instance adb14f31-274e-4dd9-8e8c-b9bad6501a74

System.Workflow.Runtime Information: 0 : pending work hc 19959204 added workItem hc 20469972

System.Workflow.Runtime.Hosting Information: 0 : TimerEventSubscriptionQueue: adb14f31-274e-4dd9-8e8c-b9bad6501a74 Suspend