Missing uart interrupts when merging peripheral_uart and multiple_adv_sets samples

Question

Setup: 
 
 nrf connect sdk version v2.1.2 
 board: nrf52840DK 
 Trying to send a beacons for half a second whenever I get some specific data on uart, and also send data over ble connection to the connected device. 
 
 Problem: 
 
 Inside uart callback function the event UART_RX_RDY: does not happen as often as it should (I assume it should happen for every received byte). 
 If I comment out all the beacon releated parts I still miss UART_RX_RDY events. 
 If I comment out all the beacon releated parts and change CONFIG_BT_EXT_ADV_MAX_ADV_SET from 2 to 1 , I start getting all the UART_RX_RDY events. 
 If I comment out all the connectable advertisment releated parts, i.e disable ble connection and only advertise a beacon whenever I get specific data on uart I start getting all the UART_RX_RDY events. 
 
 some parts of uart callback relevant part (uart callback function is almost the same as in peripheral_uart sample): 
 case UART_RX_RDY:
 LOG_DBG("UART_RX_RDY");
 buf = CONTAINER_OF(evt->data.rx.buf, struct uart_data_t, data);
 buf->len += evt->data.rx.len;

 if (disable_req) {
 return;
 }

	if (
 (evt->data.rx.buf[buf->len - 1] == '
') ||
 (evt->data.rx.buf[buf->len - 1] == '')
 ) 
 {
 disable_req = true;
 uart_rx_disable(uart);
 }

 break;
	case UART_RX_BUF_RELEASED:
 LOG_DBG("UART_RX_BUF_RELEASED");
 buf = CONTAINER_OF(evt->data.rx_buf.buf, struct uart_data_t,
 data);

 if (buf->len > 0) {
 k_fifo_put(&fifo_uart_rx_data, buf);
 } else {
 k_free(buf);
 }

 break;

Thread taking care of forwarding the data to beacon or ble nus: 
 void ble_write_thread(void)
{

	/* Don't go any further until BLE is initialized */
	k_sem_take(&ble_init_ok, K_FOREVER);

	for (;;) {
 /* Wait indefinitely for data to be sent over bluetooth */
 struct uart_data_t *buf = k_fifo_get(&fifo_uart_rx_data,
 K_FOREVER);
 int err = send_beacon();
 if (err)
 {
 printk("failed to send beacon");
 }
 if (bt_nus_send(NULL, buf->data, buf->len)) 
 {
 LOG_WRN("Failed to send data over BLE connection");
 printf("Failed to send data over BLE connection");
 }

 k_free(buf);
	}
}

K_THREAD_DEFINE(ble_write_thread_id, STACKSIZE, ble_write_thread, NULL, NULL,
 NULL, PRIORITY, 0, 0); 
 
 NOTE: 
 Even if I don't get the expected events for what ever reason, once the uart buffer fills up, UART_RX_BUF_REQUEST event happens and I can see all the data including the data for which UART_RX_RDY did not happen. So looks like the data actually gets to the uart buffer only repective event is not sent. 
 I was wondering it could be because of interrupt priorities, but I could not find documentation or any samples where interrupt priorities are modified, but I have tried the following in app.overlay without any changes in the outcome: 
 &uart1 {
	interrupts = <40 0>;
};