We have two setups (Coordinator + 16 routers and coordinator + 12 routers). All of the devices are using MKW24D512 processors.
We observed that ZigBee modules on coordinators stopped RF transmission on both networks after continuous operation of 40+ (43 and 44) days. In order to debug the issue, we used UART and we were able to communicate with the processor through ZTC command and read MAC address etc. This indicates that core of MKW24D512 processor was up and running and only RF part was having issues.
The solution we found to resolve the issue is to reset the ZigBee processor on coordinator, which is not desirable.