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Moscow, Russia? For a tropical tree? It doesn't stand a chance.
The photo of the discolored, disfigured leaf appears to be indicative of a micro-nutrient deficiency. But its impossible to diagnose without a photos of the entire tree and the site, and more information on the growing conditions, soil and water management.

Thanks for the photo. That confirms its an indoor container plant. The pattern of leaf discoloration, browning and yellowing of the tips and leaf margins indicates water deficit (drought stress). But this can be caused by root rot from over watering. So I recommend that you use a soil moisture meter and get the watering right, not to much or too little.
Other aspects of the leaf discoloration that I see is yellowing between the veins. That pattern usually indicates a nutrient deficiency.
I recommend that you find a compete organic fertilizer with a full range of plant nutrients. I don't know what's available in Moscow, but I can give you an example from the US https://www.downtoearthfertilizer.com//products/blended_fertilizer/all-purpose-4-6-2/
Notice that it has Calcium, Magnesium and Sulfur components as well as trace minerals, which most commercial chemical NPK fertilizers do not.

There appears to be a few different issues here. I am seeing a magnesium deficiency, but I think this might have more to do with the pH of the soil. But I even think this might have something to do with overwatering.

You should tip the pot on its side just a little, and see if you notice any smells coming from it. If it smell s like sulfur then there are sulfur reducing bacteria present. This will make the pH of the soil EXTREMELY acidic causing the symptoms that you are seeing.

I would recommend removing this and starting with fresh soil designed for fruit or citrus. This will be a much lighter mix and will let the plant dry out a bit. between watering.

This article will help you to care for these: https://www.gardeningknowhow.com/edible/fruits/jackfruit-trees/growing-jackfruit-trees.htm

It appears to be a minor leaf fungus affecting the lower leaves. But since the newer growth looks good I wouldn't worry about it. Just snip off the infected leaves to limit dispersal of fungal spores.
It's not water deficit, or the upper leaves would suffer first. Be careful not to over water, jackfruit trees are subject to root rot.

https://www.gardeningknowhow.com/edible/fruits/jackfruit-trees/growing-jackfruit-trees.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm

I'm looking out my window at our jackfruit tree in the garden here in Thailand. It's about 3 years old and 3 meters tall and just growing its first fruit.

It has, likely, slowed the tree down. It still has a chance, though. Just make sure to fertilize once per year, and even mulching around it can help.

It can be hard to tell without a photo. Please include one so that I can see the damage. This will help me to give you the best options for fixing the issue.

In the meantime, this article will give you information on the care of these: https://www.gardeningknowhow.com/edible/fruits/jackfruit-trees/growing-jackfruit-trees.htm