I try to growth Japanese maple from seeds at home.
After few months stratification some of the seeds sprouted.
Moreover, I have had a few small sprouts with foliage (see picture).

However, in 1-2 months the leaves began to dry and felt down.
Could you please help me to understand my mistakes and what the problem is?

Thank you very much

I think you are trying too hard...I know someone that pulls all the seedlings that come up when her trees drop seed.
If I'm quick enough, I can take some of those seedlings home and plant them in my garden.
So... I'd suggest not doing all that work.
When the trees are just out in the garden.... no effort is required.
are these your trees that the seeds come from?

Garik, Your problem is the plants are unable to take up water. Roots may be rotting. Outside, hard ground is good but in pot culture, much more air is needed to keep the plants happy. It may be too late to help them but I would try more perlite mixed in the soil. It gives the roots the air they need. More higher light levels might help but not direct sun.
You done better than me in growing them from seed. I have them all over my yard from my trees but never able to get the seeds to germinate. I must not be cold treating them right. The wild ones all over my yard is hard to transplant. But I have so many that 1 in 10 is still a lot of trees for me.

Its winter. Even inside, the little JM's will try to go dormant. But, if you are keeping them to warm, they can't do that and start to suffer. They might do better outside. Photo #3, that tree is dead. Are you keeping the soil damp? Good air circulation and cooler temperatures.

That's a good point Daisy. Young plants may be too weak to winter rest. But that may be just whats going on. Mine was looking dead and I left them outside. We'll see.

The stems should be supple and bendy with tiny nodes. The plant in photo #3 is dead, you can tell by the dehydrated look to the stem and lack of healthy nodes (and mold ).

You need more drainage and less water to keep from over watering your plants. I like starting my young seedlings and plants in smaller containers to better manage watering. The difference between your container and mine is I water my container with 75 mls and your container takes about 2000 mls water volume. So the container size you use has a very large water volume capacity. Thats why your bottom plant is in trouble, too much water in the root zone. .

Start all seed with rain water. Collect and save it. You have minerals on the rim of the containers. This is a sour medium. The media is holding too much water, you need more drainage. Get them outside as soon asap. Put them in cinder blocks. This is an inexpensive tree seedling container. Using these blocks will give you the best drainage, even in the media your using.