@pmh After a bit more testing, the workaround does allow “novnc” to work but the colour order appears swapped from RGB to BGR. Not surprisingly, this is true for both TrueNAS CORE and FreeBSD. Perhaps this was something iX had fixed by introducing their vncserver bhyve option.
I see iX has added a related post in the BETA.2 thread TrueNAS CORE 13.3-BETA2 is Now Available - #73 by Captain_Morgan