When you take the cores to a higher altitude, which causes them to dry out, it shouldn't cause them to become any weaker. In the contrary, the tensile strength of cardboard or paper should increase as it dries out. Or at least that's what I think.
It's surprising what kind of a difference air pressure differences can make. In the summer of 2002, I visited Austria. One day, a couple of friends and I took a cabin lift from a valley (at 500 m) to a mountainside (1600 m). I bought a bottle of water and we continued climbing until we came to the top at 2140 m. By that time, the bottle made up of thin plastic was empty. I screwed in the cap at the top and we proceeded to descend. By the time we had reached the valley, the bottle had become very noticeably deformed. It had lost, if my memory serves me correctly, about one fifth of its volume.
My guess is that some amount of air is trapped between the core and the pallet wrap. Because the core is much softer than the hundreds of layers of wrap put together, the air will cause weak spots in the core to give in.
5900 feet equals about 1800 m. That's even more than the difference in altitude between the top of my Austrian mountain and the valley floor.
Air getting trapped between the core and the wrap would explain the randomness of the locations of the bubbles on the inside of the core.
Possibly that could be solution to why. I have checked with a manufacturer in Johannesburg for the reasons and the answer was that the cores take 4-7 days to cure. The glue has to dry out to give it it's strength. I don't think they are even waiting 2 days before using them which would make sense. The cores are collapsing where the glue is still wet. Interesting problem but also frustrating. I like your story though and hope this isn't the case.
That story makes a lot of sense. Perhaps what you're looking at is caused by both as it doesn't happen in Durban.
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I think a thicker tube with the correct curing time will be ok. The manufacturers in Johannesburg don't have the same issues. However saying that I saw a competitors roll this week and there was signs of bubbling, but not as bad. I was hoping you were going to answer as you work things out.