I have heard few who had such encounters and I have a reason for this. Rubber that has been used for long time are degrading (regardless dry or wet),lost tension and breaking down. Like paper, rubber will also absorb and loose moisture as you use and store. When moved from a moist to dry environment for extended period, the high loose of moisture causes compaction of the rubber and hasten the crack eventually. However with mid RH, the rubber will also breakdown but turn sticky instead.
Why new rubber do not encounter this is due to the oil protection but will gradually lose with cleaning, expose to sun and perspiration. Which is why, apply wax on new rubber prolongs the livespan significantly. *BUT what is basically true about natural rubber, paper and leather is that too dry and environment will cause crack or brittle.
As previously written, when rubber, plastic touches silica gel, it will turn white, this is bec the silica absorb the surface wax on contact, resulting in permanent discoloration. This reason is why all items must be separated from touching silica gel.
Why plastic mount including interior of the lens will never outlast steel in heavy use is bec plastic is soft and flakes with abrasion even in presence of grease. The more it flakes the more the particles cause abrasion.
Those who swims in pool with watches may have encounter silicon straps turning white fast and crack. This is caused by chlorine.
I think the problem with really low humidity levels has more to do with the rubber (grips, etc) than the grease. I personally killed a vectis weathermatic by storing it in low humidity. The rubber cracked and there was some oily residue on the rubber parts. I left it in a dry box w/o a hygrometer for >6 months so I am not sure about the specific conditions needed to dry up rubber.
