I have set "Stop on Idle" true, and I see in the logs that Jenkins attempts to terminate a machine, but this is the error I get:

Failed to terminate node : oci-compute-6686a1ec-dee1-4493-b72f-243fdee78df2
2022-05-12 20:06:53.183+0000 [id=1825672]    INFO    c.o.c.b.j.BaremetalCloudInstanceMonitor#execute: ERROR : com.oracle.bmc.model.BmcException: (404, NotAuthorizedOrNotFound, false) instance ocid1.instance.oc1.phx.anyhqljrraxnh6acgjsjpmneb66k6tckya3v2mtgwrl35y2tb3465uekzbrq not found (opc-request-id: 971859E21FA8453792A755D8587282B8/9B060DFE41D115A77EF9366F36CEB555/4438D1F076FC0A09F99FA77AA57D0E4A)

I checked that the user that I used for OCI plugin is authorized to terminate vms. It also can create the machines without any problems.