Andy,
The "squishy" feel is ground water. Some pools have it, some don't. Mine does, for 9-10 years before I changed the liner this year and now my NEW liner has it. I've proved it's ground water by sampling the water and comparing to the pool. My pool has corregated drain pipe around the perimeter but it basically protects the walls.

I've evacuated nearly all the water out from behind the liner and had a tight fit but it always got squishy in the corners, along the coves within days.

You'll still get plenty of life out of the liner, I only changed ours because it had faded from over chlorination (another story).

Hope this helps, Tony