Welcome to RAK forum @kelis ,
Joining a LoRaWAN Network Server is a procedure used on OTAA mode. This means DEVEUI, APPKEY and APPEUI/JOINEUI needs to be provided to generate the session keys APPSKEY and NWKSKEY. In ABP, joining procedure is not needed.
I haven’t used a Murata module but I suspect it is on ABP and not in OTAA that’s why you do not need to rejoin. Also, the uplink and downlink counters might be disabled as well on the LoRaWAN Network Server side. If not disabled, the modules needs to store the counters in non-volatile memory and used those on succeeding ABP uplinks.
Can you share what Network Server you use? As well as the version?
As for the question if there’s a way to send without joining again, it theory that’s how LoRaWAN end-device should work. A device should only join once in its operating lifetime. However, it is very common to modules sold in market to rejoin whenever power restarted. That’s why end-devices and modules just go to sleep and not shutdown (power supply voltage disconnected).