My money is on it being corrosion.  For whatever reason, they coated the wiredbonds(sometimes) going to the IC on those, but never the display.  And those are lead wire-bonds, so they can corrode from a strong fart smell in the air 

 .  But seriously, just the least amount of battery leakage, even apparently the acidic gases can send those things into a puffy mass.  I'll be glad if I am wrong.  And be careful while under there, many get ruined by the plastic cover banging the bonds on the way off. 
 
 
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