Landscape graphic bug

Hello guys,
my level designer is blocked cause a bug formed, after the project version upgrade.
He was working with 4.11, but actually we moved on 4.13; but after upgraded it, we found some graphical bug.

Image 1
Image 2

As you can see, there are some landscape sections transparent with long distance. Looks like it doesn’t load some sections lods, because if we go close to them; they will appear.

Thank you for your attenction, hopeful in your help.

Are there errors in the log?
Has he rebuilt lighting and re-cooked?

there are not errors