By adding prints, I know that ad_uart_read() is blocking on this line:
OS_EVENT_WAIT(device->bus_data->event_read, timeout);

Some details about how I'm configuring the UARTs (UART1 for communication, UART2 for printf)

in main.c:
static const gpio_config gpio_cfg[] = {
HW_GPIO_PINCONFIG(HW_GPIO_PORT_1, HW_GPIO_PIN_3, OUTPUT, UART2_TX, false),
HW_GPIO_PINCONFIG(HW_GPIO_PORT_2, HW_GPIO_PIN_3, INPUT, UART2_RX, false),
HW_GPIO_PINCONFIG(HW_GPIO_PORT_3,HW_GPIO_PIN_2, OUTPUT, UART_TX, false),
HW_GPIO_PINCONFIG(HW_GPIO_PORT_3, HW_GPIO_PIN_3, INPUT, UART_RX, false),
HW_GPIO_PINCONFIG_END
};

..
hw_gpio_configure(gpio_cfg);

in custom_config_qspi.h:

#define CONFIG_RETARGET
#define CONFIG_RETARGET_UART HW_UART2

#define dg_configUART_ADAPTER (1)

//#define dg_configUART_SOFTWARE_FIFO (1)
#define dg_configUART_RX_CIRCULAR_DMA (1)

#define dg_configUART1_RX_CIRCULAR_DMA_BUF_SIZE (100)
//#define dg_configUART1_SOFTWARE_FIFO_SIZE (100)

Note that I've tried switching between using software FIFO and DMA and I get the same behaviour either way.

in platform_device.h:
UART_BUS(UART1, SERIAL1, HW_UART_BAUDRATE_19200, HW_UART_DATABITS_8, HW_UART_PARITY_NONE,
HW_UART_STOPBITS_1, 1, 1, HW_DMA_CHANNEL_1, HW_DMA_CHANNEL_0, 0, 0)

UART_BUS(UART2, SERIAL2, HW_UART_BAUDRATE_115200, HW_UART_DATABITS_8, HW_UART_PARITY_NONE,
HW_UART_STOPBITS_1, 0, 1, HW_DMA_CHANNEL_3, HW_DMA_CHANNEL_2, 0, 0)

in mytask.c:

ad_uart_init();

uart_device uart = ad_uart_open(SERIAL1);

for (;;) {
int len = ad_uart_read()
...
}