I've run into a lot of >500K resistors open in SMPS "start" circuits - where an IC gets its start Vcc from the rectified mains and divided down to 2.5V with half a dozen high value resistors and one low value resistor at ground. The same arrangement was also used to monitor the output and use the divided voltage for feedback. I can't recall seeing a high value smd resistor open with low voltage across them.
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>Whenever I see any inop SMPS supply with no blown output devices, I look for these first.
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>Interestingly, I've also found these resistors can be bothered by the adhesive beneath them and read *lower* value than their printed values. When I run into a supply that drifts, I remove these resistors, clean the adhesive off the board beneath them, and reinstall them. They read on the dot after that.
Adhesive is used when SMDs are wave-soldered, or for inverted or repeated reflow. All those processes require additional skill and technical knowledge to succeed, with a reasonable service life.
There are a lot of other factors influencing parasitic terminal leakage in those cases. You'd have to measure the part and the board, before and after cleaning, before and after each stage of rework, to get any idea of the leakage source.
Reduction of R in higher voltage applications is also associated with tracking and bridging of the part's element and board surfaces. All of those were affected by your simple before and after comparison.
RL