So something unexpected is going on: I can launch vnc with the configured (in /etc/vnc.conf) geometry, 3440x1440:
$ xrandr
Screen 0: minimum 32 x 32, current 3440 x 1440, maximum 32768 x 32768
VNC-0 connected 3440x1440+0+0 0mm x 0mm
3440x1440 60.00*+
1920x1200 60.00
1920x1080 60.00
1600x1200 60.00
1680x1050 60.00
1400x1050 60.00
1360x768 60.00
1280x1024 60.00
1280x960 60.00
1280x800 60.00
1280x720 60.00
1024x768 60.00
800x600 60.00
640x480 60.00
When I connect with RealVNC on Windows or Android, it gets the right high resolution. And it survives past disconnects and reconnects. However, if I connect with the vncclient command on my Linux laptop, it switches to the lower laptop resolution (1600x850 or something like that)...and even if I disconnect the session and reconnect and specify "vncclient -geometry 3440x1440 :0", it still goes with the lower laptop resolution, which, and this is the part that irks me, then somehow gets picked up by RealVNC next time I connect from it. The 3440x1440 mode disappears from the xrandr output as well. I have a script to add in the mode line and set it in xrandr as a workaround, so this isn't exactly high-impact to me, but I was wondering what was going on here. The logs don't give any indication of anything.
It's a default setting in the TigerVNC client to set the remote server to local geometry. Press F8 and unselect the option. I don't see a command line option that overrides this.