Yep, this confirms that the ref and the amp are working correctly.
1.7Ω is a lot of resistance; if it exists on your board you should be able to find it with a DMM.
If it isn't there, it has to be in the chip.
(We already know that it exists somewhere between the internal feedback divider and your jumper.)
This means load regulation from this 10V/-5V output is worse than TL431
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif.pagespeed.ce.5xOqKkq0Co.gif)
edit
Theoretically, a different explanation is possible too. If input bias current of the amp is significant and changes with output load current, then BUF_S voltage is no longer half the output voltage and things get weird. But let's do the math: with 1mA load current, 10V output decreases 1.7mV. BUF_S is constant, so current through the upper 9.3kΩ resistor decreases 0.2µA. This would be caused by 0.2µA change in input bias current, which seems very unusually large.
I don't even think it's worth trying to modify the circuit to test this theory. I'm running out of even the most crazy and far-fetched alternatives to the simple explanation that there is 1.7Ω in series with this output not compensated by feedback.