Outbound Campaign Sync Fails with 400 Bad Request During WFM Peak

Can anyone explain why the /v2/outbound/campaigns endpoint returns a 400 Bad Request when syncing lists during our America/Chicago weekly schedule publish?

Background

We run high-volume outbound campaigns tied to WFM shifts.

Issue

The API rejects valid list IDs with invalid_campaign_list error only when concurrent API calls exceed 50/min.

Troubleshooting

Verified list validity via UI. Errors disappear if we throttle requests to 20/min. Is there a hard rate limit tied to WFM events?