WFM API 400 on Schedule Group Import via CLI

Can anyone explain why the WFM schedule group import fails with a 400 error?

Background
Using Genesys Cloud CLI v2.1.0 in AU-1 BYOC. Automating WFM config via Terraform and CLI scripts.

Issue
POST /api/v2/wfm/schedules/schedulegroups/import returns 400 Bad Request. Payload is valid JSON.

Troubleshooting
Validated JSON structure. Checked timezone offsets. Error message is generic ‘Invalid input’. No specific field highlighted. Any hints on strict schema requirements?