I too have several hundred flip-tops, most of them Fischer, and I have never had any problems with them. The glass is quite thick. You made a good score.
As for the gaskets, I have never seen nor heard of them being glued. Logically, that would reduce their effectiveness since it would make them less flexible. I think you just have some that were not cleaned and the residual sugar from the beer has solidified. My advice is to soak them in hot water to loosen them up. If any gaskets are stiff, hard, or have cracks then replace those. I have some gaskets that have been reused many, many times, but I replace them if anything looks or feels wrong. I always rinse them well every time I empty a bottle.
Incidentally, I don't remove them from the tops every cleaning, only if they look bad. I put the entire bottle in hot water and PBW to clean them, and I submerge them in mild bleach water to sanitize (and rinse well afterwards). I have never had any infections. However, your cleaning and sanitizing procedures may make removal a good idea.