From the photos I would suggest cutting the foam down at that edge and putting the liner back in the track. Over time the liner track itself fatigues and flexes a bit – resulting in a sagging liner or it falling out as it did. A common trick is to put pennies in to hold the liner in place in lieu of locking strip which is sometimes effective and with older pools/retro fits the pennies work better at keeping it in place. The liner should be carefully stretched back in on a sunny day when the liner is more flexible. Avoid using tools that could cut the liner – sometimes the water needs to be lowered a bit to make this happen. I have also never heard of “liner adhesive” so not sure what they would have been referring to?