This happened on OpenWrt SNAPSHOT r13520-68b94f0fb4, but it's pretty random. I haven't been able to figure out what causes it. It usually happens at least once within a week. I'm not sure why there's no call stack here, I also tried cat /sys/kernel/debug/crashlog a few hours after it happened and got a "No such file or directory".
# uname -a
Linux EA8500S 5.4.43 #0 SMP Mon Jun 8 19:16:17 2020 armv7l GNU/Linux