In one of my projects nanites works with trees and other vegetation, but in another project with the same version and the same settings, they do not work, what could be the problem?
At the moment, in fact, Nanite doesn’t work with masked materials and world position offset. Foliage uses both of these, so I’m not really surprised 
Yes, you can turn it on, because it’s a static mesh. But you’re getting no benefit from it.
If it wasn’t an issue, why would products like this exist?
I feel stupid. but it seems that this product does not support nanites, but they only work in one scene and i can’t turn them on in another Megascans Trees: European Black Alder (early access) in Megascans - UE Marketplace
Notice how they’re talking about LODs in the bump, which they wouldn’t if it was Nanite.