ConfigMap volume is sometime delayed when mount it as file

Did any one know that ConfigMap volume maybe delayed occasionally when mount it as a file. We met a problem that our application process in pod can’t read configuration file from ConfigMap when startup because the file did not exist at that time, we fixed it with the workaround of waiting the file some time in process when starting.
As I understand, the volume must be mounted before container startup in kubernetes, so this problem must not happen, maybe this is a problem in k8s.

Cluster information:

Kubernetes version:
Major:“1”, Minor:“21”, GitVersion:“v0.21.0-beta.1”