I've done several references using SMT precision resistors. What I 
learned- the hard way- is that I got into problems whenever I undersized
the 
precision resistors.
If calculations indicate that a 1206 will be fine, use a 2512 instead. 
Ensure that the full contact surface is actually soldered down, so that
self-heating is pulled away from the precision resistor. Keep the
precision resistor away from other heat sources, too.



It's as far away as practical, with a ground plane layer below to spread
the heat. 


While contamination is possible, I am inclined to think overheating is 
the problem.

Worst case dissipation would be 9.5 V into the series combination of
332k and 100k, so the 332k gets a grand total of 160uW.  Seems within
tolerance for an 0603?




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