After the initial aging, let's say until day 200, we can see about 3.3 or 1.8 ppm drift in 800 days, the average of 2.55 ppm being the most likely result. So roughly 1 ppm per year.
I think that's a normal result for an ovenized, buried zener reference including a 7 V to 10 V gain stage. To be honest, analysis requires recalibration of the other zener references as they can drift at a similar speed.
Regards, Dieter