Ah, I’ve just checked my material and yes, it’s the problem. The problem was in my Material Layer Blend settings where I mistakenly set the opacity mask outside the polygon to zero. This action cleared all opacity from the underlying layer. Moreover, the layer beneath it also cleared all opacity outside its area, rendering it invisible as its display zone was outside the upper layer.
To rectify this, I’ve made some adjustments. Instead of breaking all masks outside the polygon, I’ve decided to keep the material outside and only break the region within this polygon. This change will result in a terrain with multiple gaps, which are the regions I intend to preserve. The next step is straightforward - I just need to invert all the masks.
For the top layer, I’ve created a new Invert Mask Layer Blend that inverts everything and displays the region of interest.
And now my region appears!
I hope this explanation is helpful for someone.