Except there are no active connections on your CloudSQL Dashboard during the selected period, there really should be no reason for the missing data. Hence, for better troubleshooting of the issue, which may involve reviewing configurations on your Project, I recommend you contact the GCP Support Engineers[1] or if you think this issue is a bug, then open an issue using this link[2].
This way a GCP Support engineer would be able to work closely with you, reviewing your application configuration and determine exactly what may be the cause.