As mentioned we've seen this often, and without anything connected to the SDM3065X except the reference and with other references, so it's definitely coming from the SDM3065X!! Don't see this behavior with any of our other seven total 6 1/2 digit DMMs, just the SDM3065X.
Agree, 1/2 ppm not too bad, 1.5 ppm is concerning and at some point may consider changing the LM399.
Hopefully some folks will report what they "see" with their SDM3065X DMMs.
Best,