I’m getting this issue in 5.6 as well. Our landscape material had 1 weight blended layer, with the rest being height blended. Changing the weight blended layer to a height blend (with a height of 0.5) is a simple workaround in our case.
Strangely the issue seemed to disappear for a while after I moved the landscape asset to a different folder. However, after painting one of the height blended layers it appeared again.