When the Event-Timestamp Attribute is present, both the Dynamic Authorization Server and the Dynamic Authorization Client MUST check that the Event-Timestamp Attribute is current within an acceptable time window. If the Event-Timestamp Attribute is not current, then the packet MUST be silently discarded.
Check the time on both EXOS switch and NAC and see if the they are in sync. If the time difference is more than 300s per RFC, adjust the time on both ends or synchronize the time using NTP
Another reason can also be a misconfigured radius dynamic-authorization secret. In that case nothing will be logged and the EXOS switch will just ignore the CoA request. There is no way to verify this, reconfiguring the secret would be the only to be sure.
configure radius dynamic-authorization 1 server 10.116.2.202 client-ip 10.116.3.48 vr VR-Mgmt shared-secret <secret>