Running top -m io (or pressing m interactively) shows data initially but upon first refresh read/write columns are always zero. VCSW, IVCSW and occasionally FAULT show data properly.
Tested and reproduced under 12.0 release, U1.1 and U2. Tried 3 different desktop class machines and a hyper-v virtual machine.
Tested under 11.3 and data shows properly. Tested under FreeBSD 12.2 and data shows properly indicating TrueNAS specific issue.
zpool iostat 1 shows plenty of IO at the volume level but unable to troubleshoot at the process level.