I can prove it … 
On your TN 13.3 beta edit /usr/local/lib/python3.9/site-packages/middlewared/plugins/vm.py, go to line 922 that looks like this:
f'vncserver={attrs["vnc_bind"]}:{attrs["vnc_port"]}',
Duplicate the line, place a comment sign in front of the copy, edit the other one so now the two lines look like this:
f'tcp={attrs["vnc_bind"]}:{attrs["vnc_port"]}',
# f'vncserver={attrs["vnc_bind"]}:{attrs["vnc_port"]}',
Clear the python cache (not sure this is strictly necessary):
rm /usr/local/lib/python3.9/site-packages/middlewared/plugins/__pycache__/vm.cpython*
Restart middlewared:
service middlewared restart
Restart your VM. Look up the VNC port from the UI. Use an external VNC viewer to connect to <IP of your NAS>:<VNC port>
Voila! Plain bhyve VNC without extras just works.
This procedure breaks the builtin web VNC viewer but it clearly shows that the problem is with some addition(s) by iX and not with bhyve.
I’d rather (similar to the shell) have no web VNC than no VNC at all.
Kind regards,
Patrick