There' a good chance the original builder encountered ground water upon construction.

Generally a pipe is set at the deepest point, below the finish and is run up to the patio height. The end of the pipe in the deep section is perforated and surrounded with gravel. This enables a typical pool pump to be attached to draw the groundwater out during construction to keep things dry.

Sometimes the pipe is hidden under a diving board base or at the back of the patio.

In some extreme cases, a well point is set up with a pump, to constantly keep the ground water at bay.

Performing that may remove the groundwater.

You could add water back to the pool in an effort to "equalize" the pressure so the liner can go back. Problem is, you'll have to monitor the water level before things really freeze over. Also, if the liner does recede, there's good chance it will have some wrinkle in it.

Draining the pool will most likely make things work and allow more groundwater to enter and the liner to float even more.

Also, the liner will shrink enough that it will not stretch back into place. It can tear or pull out of the bead receptor at the top.

Your water level in the pool going into freezing temps, should be a little below the skim face plate.

Thank you for the explanation. the french drain looks empty. we will add more water and see if it goes down.