CUSTS1_VAL_NTF_REQ message duplicated

I've a problem if I send CUSTS1_VAL_NTF_REQ messages too fast. If I send e.g. 3 different messages (e.g. "A", "B" and "C") the SDK is sending 3 to the central, but the central receives often the following messages "B", "B", "C" or "B", "C", "C". I verified this with a sniffer, but the problem also is visible if I use android as a central or a Rasperry Pi. There is no assert thrown in the SDK or an error thrown on any function.

for(int32_t i = 0; i < 3; i++) {
// setup buffer ...
struct custs1_val_ntf_req *req = KE_MSG_ALLOC_DYN(CUSTS1_VAL_NTF_REQ, TASK_CUSTS1, TASK_APP, custs1_val_ntf_req, length);
req->conhdl = app_env->conhdl;
req->handle = CUST1_IDX_SERIAL_RX_VAL;
req->length = length;
memcpy(req->value, buffer, length);
ke_msg_send(req);
}