nrf_serial_read() timeout never triggers when called from APP_TIMER callback

Question

Hello, 
 While building a custom board, I need 52832 to communicate with a modem at regular intervals, send AT commands and read modem responses. nrf_serial library in IRQ mode seems like a good choice, since it can put CPU to sleep while waiting for the response, and also support Rx timeouts. 
 Although nrf_serial_read() works as expected when called from the main thread, calls from APP_TIMER never return. It seems that: 1. nrf_serial_read() busy-waits and never goes to sleep, 2. serial_timeout_handler() never fires so nrf_serial_read() keeps busy waiting. 
 Playing with priorities didn't help much. I am using SDK 15.2 with softdevice enabled. Below you can find how to reproduce the problem, can you please assist? 
 #include "nrf_serial.h"
#include "nrf_pwr_mgmt.h"
#include "app_timer.h"

void test_serial_evt_handler(struct nrf_serial_s const *p_serial, nrf_serial_event_t event);
void test_idle_state_handle(void);

NRF_SERIAL_DRV_UART_CONFIG_DEF(test_uart0_drv_config, 
 8, 6, 0, 0,
 NRF_UART_HWFC_DISABLED, NRF_UART_PARITY_EXCLUDED,
 NRF_UART_BAUDRATE_115200, 5/*UART_DEFAULT_CONFIG_IRQ_PRIORITY*/);

NRF_SERIAL_QUEUES_DEF(test_serial_queues, 128, 128);
NRF_SERIAL_BUFFERS_DEF(test_serial_buffs, 8, 1);

NRF_SERIAL_CONFIG_DEF(test_serial_config, NRF_SERIAL_MODE_IRQ,
 &test_serial_queues, &test_serial_buffs, test_serial_evt_handler, test_idle_state_handle);

NRF_SERIAL_UART_DEF(test_serial_uart, 0);

APP_TIMER_DEF(test_Tx_timer_id);

/*
*/
void test_idle_state_handle(void) {
 nrf_pwr_mgmt_run();
}

/*
*/
void test_serial_evt_handler(struct nrf_serial_s const *p_serial, nrf_serial_event_t event) {
 // do nothing
}

/*
*/
void onTestTxTimerTick(void *data) {
 // try to read 10 bytes in the next 1 second
 uint8_t inBuf[10];
 size_t bytes_read;
 nrf_serial_read(&test_serial_uart, inBuf, sizeof(inBuf), &bytes_read, 1000);
 
 // Rx pin is unconnected, so we should return with a timeout. 
 // Unfortunatelly, we never reach this point !
}

/*
*/
void test_main() {
 APP_ERROR_CHECK(nrf_serial_init(&test_serial_uart, &test_uart0_drv_config, &test_serial_config));

 // create a 10-sec repetitive timer
 APP_ERROR_CHECK(app_timer_create(&test_Tx_timer_id, APP_TIMER_MODE_REPEATED, onTestTxTimerTick));
 APP_ERROR_CHECK(app_timer_start(test_Tx_timer_id, APP_TIMER_TICKS(10000), NULL));

 // Enter main loop.
 for (;;) {
 test_idle_state_handle();
 }
}
 
 Thanks, 
 Thanos