It does sound like the installed udev specification was written on one type of Jetson, and as you say, it would blindly assume the particular device tty. Whoever wrote the original udev spec for the device did not take into account any Jetson model other than the one the udev script was for.
You will probably find the “udevadm” tool of use. The tool can provide information which udev is capable of using. So for example, depending on which system you are on, try one of these for more information about the particular device:
sudo udevadm /dev/ttyTHS0
sudo udevadm /dev/ttyTHS1
Perhaps the “path” (“DEVPATH”) could be used to differentiate. Can you verify you want “ttyTHS1” on the Nano, but device “ttyTHS0” on the Xavier NX? If it turns out that the path for ttyTHS0 and ttyTHS1 are the same, then this won’t help much, but if one of the platforms uses a different path for a specific ttyTHS0 or ttyTHS1, then you’ve found a way to tell udev which to use (you’d go by path instead of the name “ttyTHS0” or “ttyTHS1”, though you could specify both device and path). Basically, run the above “udevadm” commands for both ttyTHS0 and ttyTHS1 on both systems, and look for a difference.