Not clear if this is related, but after upgrading from 5.5 to 5.7, loading the level, then restarting the editor, upon open the landscape was below the ocean and flat.
Deleting the landscape ‘Water’ layer fixed it back to the original landscape, perhaps this layer has to be setup again.
This issue may be related Unreal Engine Issues and Bug Tracker (UE-225683) - has a target fix in 5.6, but the project upgrade skipped the 5.6 version when updating from 5.5 to 5.7.
Upon removing the ocean layer the landscape was again rendered as expected.
See also Landscape Layers in 5.7 Not applying correctly - #4 by unit23