Hard fault by using UART? nRF52832

Hello.
I am using nRF52832, SDK ver 15.3 and running it as a peripheral device.
I use an interval timer to send a UART to an external device and receive an ACK.
After running it for a while, HardFault_Handler will be called as shown in the attached image. (When I look at CallStck, it says tx_power_set(), but this function is only executed the first time.)

Also, the shorter the UART transmission interval is, the earlier HardFault_Handler is fired in proportion to it.
For example, if I set the UART transmission interval to 100 msec, it will hard fault in 1.5 minutes, and if I set it to 50 msec, it will hard fault in about 40 seconds.
The number of UART transmission bytes is 10 bytes, and malloc is not used.
Heap and stack capacity are the defaults.
In addition, after sending UART normally, HardFault is occurring before a reception event occurs.

I use UART transmission function is nrf_drv_uart_tx().


I'd like some advice on what to look into next.
Thanks in advance.

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