PATCH /api/v2/wfm/schedules/{scheduleId} 409 Error on Shift Swap Overlap

Can anyone explain why the PATCH endpoint is throwing a 409 Conflict when applying a shift swap that technically fits within the agent’s available time off in the America/Chicago timezone? We are running Genesys Cloud 2024.6 and the payload below is valid according to the schema, yet the publish job fails immediately upon detecting the swap.

{
 "shifts": [
 {
 "date": "2024-05-20",
 "type": "swap",
 "agentId": "a1b2c3d4-e5f6-7890-g1h2-i3j4k5l6m7n8",
 "startTime": "2024-05-20T08:00:00.000Z",
 "endTime": "2024-05-20T16:00:00.000Z"
 }
 ]
}