MariaDB server hang with all threads idle

I have a production MariaDB server which is occasionally experiencing a hang; it refuses new connections, does not create any more threads, and stays at 0% cpu until forced to restart.

I'm using threadpool:
thread_handling=pool-of-threads
thread_pool_size=48
thread_pool_max_threads=128
thread_pool_idle_timeout=30

and xtrabackup. The issue always occurs around the same time of day (once every week or two) so I suspect it may be related to xtrabackup's pausing the server to backup.

Captured a gdb backtrace in this state, but imposed a workaround of restarting the process rather than try to debug further, since the server is being used.