What is a configmap cache?

From the docs here

The kubelet uses its local cache for getting the current value of the ConfigMap from Kubernetes API Server. The type of the cache is configurable using the ConfigMapAndSecretChangeDetectionStrategy field in the KubeletConfiguration struct. A ConfigMap can be either propagated by watch (default), ttl-based, or simply redirecting all requests directly to the API server. As a result, the total delay from the moment when the ConfigMap is updated to the moment when new keys are projected to the Pod can be as long as the kubelet sync period + cache propagation delay, where the cache propagation delay depends on the chosen cache type (it equals to watch propagation delay, ttl of cache, or zero correspondingly)

failed to sync configmap cache could be because kubelet could not connect to Kubernetes API Server to get the updated configMap data. I would look into Kubelet logs using journalctl -u kubelet