Zabbix Runtime Control function tell the message to child processes for Server, Proxy and Agent. Then the message size is C Language int, may be 32bits. The function slices it to 16bits data, 8bits scope and 8bits command.
Now the function for PID is implemented to increase or decrease debug level. Though in 64bit Linux PID max is for 15bits, it can be increased. So, 16bits data for PID is overflowed.
To resolve this, use void pointer (void *sival_ptr for union sigval) instead of int value (int sigval_int, i.e. ZBX_SIVAL_INT) for sigqueue() must be used on zbx_sigusr_send().