POST /api/v2/conversations returns 401 immediately after token refresh

I’m completely stumped as to why my Open Messaging bot receives a 401 Unauthorized response right after successfully refreshing the OAuth token. The clock skew between my server (Berlin) and the Genesys Cloud API is negligible.

1. Call POST /api/v2/oauth/token with grant_type=refresh_token.
2. Receive new access_token and expires_in.
3. Immediately call POST /api/v2/conversations/messaging/interactions/{id}/messages with the new token.

The request fails with HTTP 401. JSON error payload:

{
 "errors": [
 {
 "code": "unauthorized",
 "message": "Invalid or expired token"
 }
 ]
}

Is there a propagation delay for token validity checks in the backend?