Datasheet lists the variation of Vref with temperature, so you can take that into account.
That of course is not the actual SAR converter front end drift. For that you would
need to file a CASE as its not in the datasheet as you have discovered. It may be wrapped
up in the overall performance spec.
It would be great if you post back the answer you get to benefit all.
To create a technical or issue case at Cypress -
“Create a Case”
You have to be registered on Cypress web site first.