Pool only leaking in winter - two years in a row

I don't see a hydrostatic valve in the pool's deep end in the picture. Are there any penetrations in the liner lower than the pool light? How old is the liner?
To have that much water escape that quickly it should be noticeable that a small lake is forming in your yard indicating something is up.
Is there a drain under the liner prevent the liner from floating? if it's routed in a way that it can act as a siphon once it gets going there is nothing to stop it. At the same time the liner has a leak and the pool shell is good enough to hold the water.
Are there any light or returns on the steps or shallow end?
Some photos of the yard and slopes may help figuring this out. In stead of sealing off all returns and lines I would leave them open to atmosphere to limit the possibilty of a siphon from happening.
Do you have an overflow?

I don't see a hydrostatic valve in the pool's deep end in the picture.

Are there any penetrations in the liner lower than the pool light?

How old is the liner?
Not that I can see, the cover is still on. The liner was installed in 2015 so going on its 10th year. We are planning to replace but didn’t want to run into the same issue next year with a new liner. Confusing to me why it’s only leaking in the colder months and started after we got 12 hours of rain. Just trying to figure it out!
 
Main drain. If the outlet is below the water level, even if it goes up at the pad, it can drain the pool down to the level of the outlet. The primary difference in summer and winter is the way the plumbing is configured at the pad, so that seems the first place to look. Otherwise I can’t even come up with a wild guess of what could make the pool hold water perfectly during the summer, then suddenly dump a bunch at once in the winter.
Two years in a row. To almost the exact spot. I’m baffled as well. I’m not understanding what you’re saying about the outlet?
 
Is it really down to the same level? The picture with the cover looks to be higher than the first picture. How permeable is the cover? Is is suspending trapped water above the actual level of the pool? (ie - cover not actually supported by the water underneath?) A lot of work, but you may want to pump the water off the cover, so you can peel it back to see where the level actually is underneath.
Does it continue to drop, or is it stable once it gets to a given level?

Pictures showing the relation of the pool to the flooded yard/driveway areas may help a lot. Include where water goes (pipes, discharges) when backwashing or pumping to waste.
Any recurring problems with wet basement (or other house foundation water issues) at other times, or other indication of a high ground water level?

At a WAG - I'd suspect a fault in the main drain plumbing, where with suction on the line when the equipment is running, it seals things enough to make a leak unnoticeable. But with that gone, it drains. Which doesn't fully explain stabilizing at a set level, however....unless it is a crack in a place where the pipe rises up from underneath to get to the equipment pad.
 
Which doesn't fully explain stabilizing at a set level, however....unless it is a crack in a place where the pipe rises up from underneath to get to the equipment pad.
Either that, or the crack / defect is below the ground water table. The pool wants to drain more, but the water has nowhere to go. If the crack is above the ground water table, it drains to the crack. Screenshot_20241219_110118_Chrome.jpg
 
Either that, or the crack / defect is below the ground water table. The pool wants to drain more, but the water has nowhere to go. If the crack is above the ground water table, it drains to the crack. View attachment 621733
Hmmm, wonder if in the summer the defect is actually sucking water from the water table?? If evaporation never has to be countered....
 
wonder if in the summer the defect is actually sucking water from the water table??
Or its just sucking air like you said, instead of leaking if the water table is lower at that time. The pump being off for some hours (if so) may not amount to a noticable drop with most of us getting more rain in the spring and summer.
 
With the volume of water it shouldn't to difficult to determine where the water is comming from. 5000+gallons overnight is like a 1-1/2" or 2" line gravity draining. Even with a mimimal vertical drop a 1800-3000 gph flow is pretty easy for gravity draining with 1-1/2 to 2" pipe. For reference a 3/4" garden hose can do about 600 gph at 60psi. Gravity draining with a garden hose is maybe 150 gph.

When the conduit to my pool light was broken the pool only lost an inch or so water a day (about a 1000 gals).

Photos of the yard, equipment, and details of how the pool is plumbed.

Based on you first and last photo from your orginal post I would bet on the light fixture leaking. If the first photo was after the cover was drained and removed, that would raise the water level under the cover to about where the light is. I can see how it's possible that it was not that noticeable with a cover and rain accumulating on the cover making it seem like the pool was not leaking. Plus you probably don't look at it as ofter and don't look at the water level realitive to the skimmer to see if it needs to added or removed.

If you have occasional rains when the pool is open that also masks the leak. Is it possible someone else was adding water to the pool when it was open?
 
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Do you use a submersible pump to drain the water off the pool cover? If you have a hole in your winter cover you will also be draining water from the pool.I drained my pool down
over a foot lower than closing level one winter.I would just leave the pump on the cover all winter attached to a garden hose and it would siphon water out even after i turned the pump off.
This is when I lived in Maryland.
 
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