Just to have a solution here. The problem occurs if data is being received on the UART while /dev/ttyHS0 is being opened. The kernel panics and the device crashes. I don’t know if this is a Linux or a Legato but it should urgently be fixed.
Linux: Linux swi-mdm9x28-wp 3.18.140 #1 PREEMPT Tue Aug 3 08:17:34 UTC 2021 armv7l GNU/Linux
Legato: 19.11.6_af84a308b18c93dd4f3ae40f63c9bc67