Dear OpenZFS: if you are going to produce a kernel message with the string 'PANIC' in it, and then a kernel stack dump, please make it an actual panic so that my 'reboot on panic' settings kick in. Otherwise we will be very confused.
Also, upgrading old ZFS filesystems from version 4 (I said they were old) to version 5 is apparently not harmless on Ubuntu 22.04 OpenZFS.