No way to prevent that in a back pocket.

Something like that, folded, and subject to repeated stress will eventually fail at the folds.

Pinholes don't really matter, carry a bit of duct tape for field repairs.

The real answer is to move it out of your back pocket.

Where do you live that you would need to carry a heat sheet every day and not be carrying a back pack of some sort, or at least a waist pack?