OK. Looking very carefully at the photo from the teardown. Notice the volume buttons are place on a relatively thick steel bracket. Looks like the intent was to have this provide some structural strength to where the cutouts for the volume buttons are. This bracket appears to be held in place by 2 small screws that attach to the small black inserts. My take here is that probably what is happening is one of two things:
1. The small black inserts move upwards when the phone is subjected to pressure in this location thus effectively allowing the bracket to move reducing the support it gives the frame.
2. The screws holding the bracket are made of soft metal and they themselves bend or shear off when the phone is subjected to pressure at this location. Or it could be as simple as some assembly error where the wrong screws were used in a particular batch of phones.
Either way I think it's an easy manufacturing modification that can be made quickly.
The fact that these relatively thick brackets were used leads me to conclude that Apple recognized the areas with cutouts would be weakened and worked on strengthening them.
Just my 2c.
./sherif
Could those also simply be liquid inhibitors?