I know you have your setup pretty dialed in there, but if you want to protect against that (and against your cameras recording under the wrong date after x minutes of no internet) some routers like Asus can intercept all NTP packets and respond with its local time. I tested this with a Wyze cam and it worked perfectly, I disabled my WAN, rebooted the cam, it reached out to several internet based NTP servers and my router responded impersonating those servers, and the cam time synched right up.
Of course, if the router has rebooted, it will also have the wrong time, so it only helps if the router is on UPS and it hasn’t been long enough to deplete the UPS.