I have a feeling this is exactly the same problem that developed with my pool a couple years ago. That plastic piece the liner hooks on to broke at the curve in the deep end where two of them came together. Not one continuous piece. I attribute mine to the age of the pool, about 35 years. Still not sure what to do about it. Only let go for about 6" and what is there seems to be holding. I also didn't get anything behind the liner. I'll be curious what we find out. My pool is pretty well beat.
Al