Going back through logfiles the first occurrence is
Jan 29 12:27:52 donny kernel: [72122.085286] PANIC at zfs_vfsops.c:585:zfs_space_delta_cb()
So it seems to have started after I upgraded from Linux version 5.4.0-1026-raspi to 1028
Jan 27 04:27:18 donny kernel: [ 0.000000] Linux version 5.4.0-1026-raspi (buildd@bos02-arm64-062) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)) #29-Ubuntu SMP PREEMPT Mon Dec 14 17:01:16 UTC 2020 (Ubuntu 5.4.0-1026.29-raspi 5.4.78)
Jan 28 16:25:31 donny kernel: [ 0.000000] Linux version 5.4.0-1028-raspi (buildd@bos02-arm64-034) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)) #31-Ubuntu SMP PREEMPT Wed Jan 20 11:30:45 UTC 2021 (Ubuntu 5.4.0-1028.31-raspi 5.4.78)
Going back through logfiles the first occurrence is c:585:zfs_ space_delta_ cb()
Jan 29 12:27:52 donny kernel: [72122.085286] PANIC at zfs_vfsops.
So it seems to have started after I upgraded from Linux version 5.4.0-1026-raspi to 1028
Jan 27 04:27:18 donny kernel: [ 0.000000] Linux version 5.4.0-1026-raspi (buildd@ bos02-arm64- 062) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1 ~20.04) ) #29-Ubuntu SMP PREEMPT Mon Dec 14 17:01:16 UTC 2020 (Ubuntu 5.4.0-1026.29-raspi 5.4.78)
Jan 28 16:25:31 donny kernel: [ 0.000000] Linux version 5.4.0-1028-raspi (buildd@ bos02-arm64- 034) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1 ~20.04) ) #31-Ubuntu SMP PREEMPT Wed Jan 20 11:30:45 UTC 2021 (Ubuntu 5.4.0-1028.31-raspi 5.4.78)