Debian 7.8 and the LXDE desktop work with tightvncserver, although a bit fuzzy compared to a Raspberry Pi3. From 8.1 to 8.6 and the lxqt desktop the keyboard doesn’t work. The fix for GNOME doesn’t have any effect.
Use sudo apt install xfce4.
I was researching ideas. I found this post from years ago. Look here for promoting keyboard usage on the BBB with different vncserver viewers: dpkg-reconfigure keyboard-configuration
That command should allow you to fix the basic keyboard config. for your system.
P.S. Also, if it is too late, '16 to '20, I understand.