I have some more suggestions based on some additional testing. I've found that changing video modes after boot-up can lead to issues. To minimize this, I've gone to the NVCP to force a 120Hz refresh rate and also to disable any scaling. I think this will help the TV stay at 4K/120 at all times. Another approach is any time a new game is installed, load it, set it to 4K/120 in the game settings, and then exit and reboot.
Also, to prevent mode changes when changing to HDR, you need to manually set 4:2:0/10-bit in the NVCP as well (otherwise, I think it switches to 8-bit when HDR is turned off).
If I boot up in 4K/120/4:2:0/10-bit (in addition to everything I mentioned in my earlier post, around having GSync and Game mode already enabled), I seem to always be able to get the pendulum demo to work with GSync.
The above approach may not work for those of you who need better than 4:2:0 color for non-gaming situations, unfortunately, as switching between those modes seems to be a problem.