Inground pool -mysterious leak - needs help!

Hello there: It's great to join this forum.

Newbie here. Bought a house two years ago and have been stuck with an in ground liner pool (25K gallon) with steps, for the past 2 years. Have been able to resolve several issues except a chronic mysterious leaking problem -

- Have been losing water year round; it got more and more serious; right now about 1/2 inches per day.

- A guy used a "stethoscope" type of device to check the liner - found no leak.

- Dye test reviewed leaks on the step edges/ reset the steps, and poxy putty was used - pool still losing water (probably not from the steps).

- Pressure tested all skimmers, returns, and main drain / found a leaky return line on the step - cut and plugged it - pool still losing water.

- Worked with 2 companies; current one applied the putty and cut the return line above; they are suggesting us to let the water run down below the skimmers/faceplates
(I seroiusly doubt this would work as during winter, we plugged everything and water continued to lose until about 21 inches below surface. So it should not have anything to do with the faceplates.)

This is really a mysterious. I am hoping to get some advice from you guys - the fellow pool owners who have experience with leaks.

Thanks!

Paul