If not configured for device:
mac-settings.rx1-delayis assumed to be1as per the specmac-settings.desired-rx1-delayis assumed to be5on our networks
So given your device NS reads the MAC settings as:
Device is using RX delay of 1 by default, but RX delay of 5 is desired - send RxTimingSetupReq to make device use RX delay of 5.
If you want your device to use RX delay value of 1, please set mac-settings.desired-rx1-delay to 1