You can Google this and find the manual mirror creeping out is a common occurrence.
Most, like mine, will only creep out about a half inch or less and then stop. And for me, only the driver's side does it.
Haven't needed to find a fix or adjustment for it as it hasn't gotten worse in 11 years.
I suppose if it really bugs you, you could replace the mirror, but GM is certainly proud of them
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Scott and Liz - Southern NM
2012 Wildcat Sterling 32RL - w/level up (best option ever)
2007 Chevy 2500HD Duramax
Reese Fifth Airborne Sidewinder