Landclass of a slope between 2 part of terrain, that has different elevations (about 30meters and higher) automatically changed to a green forest ( if elevation difference is about 500m - then it changes to rock). Does it common issue, hardcoded inside fsx or I've got something wrong with my sim? (FSX Acceleration)

This is correct, there is a gradient map that changes the land class assignment as elevation increases:There is no simple solution, although it is possible to get around the problem. For example, some land class assignments do not change with elevation (for example, rock ice, lava, reef, etc.) So, you could use those values for your projects, and then include the desired ground textures renamed to the ice or lava names and placed in the local scenery folder. FS will think that those are the textures it needs.Best regards.Luis