You mentioned Windows already booted up first: Did you mean client (Windows) or a DC ?
Ideally, if the DC is up first, and then Filer boots up, most ideal situation. cifs access should be ok as long as the time difference is not skewed (You have already verified this).
However, if you meant the client (Windows) : That's a different thing altogether.
Windows Client : May have a stale connection and by rebooting you fixed it 🙂
As a troubleshooting step: You could always check few things.
1) ::> cifs session show [sessions on storage]
2) c:\> net use [sessions on client]
Just compare it : If sessions are available on client but not listing out in storage then those sessions are stale and will not be accessible.
Thanks!