DeviceOS 2.x introduces an option to wake up on new data arriving on network interface, so your sleep configuration (.network(NETWORK_INTERFACE_CELLULAR)) actually instructs to do just that, hence the BY_NETWORK wake-up reason.
The SLEEP_NETWORK_STANDBY is synonymous to .network(NETWORK_INTERFACE_CELLULAR, SystemSleepNetworkFlag::INACTIVE_STANDBY), which will leave the network interface powered and connected but in paused state so that the modem can potentially cache any arriving data while we sleep, but will not configure it as a wake-up source.