Small update. We have found to get much better results when we use traditionally mipped textures in our landscape material (specifically the part that writes into the Runtime Virtual Texture), as opposed to using Virtual Textures (that have “Virtual Texture Streaming” ticked in the texture’s details panel).
It looks like Virtual Textures are not currently supported when writing to the Runtime Virtual Textures. They “work”, as in it won’t crash, but the behavior is erratic