Not able to create Custom Disk Detection rule with 0% threshold using API. However, it is possible using UI. When threshold is passed as 0 in payload of POST api/config/v1/anomalyDetection/diskEvents API, the rule gets created with empty threshold field in UI.
Are there 2 sections where you need to update it? I know for custom metrics there are and for it to update correctly the values must match. If the issue is still occurring after that then I'd recommend opening a support ticket.