Would be grateful for any input.
Did it always happen? Did you set your landscape collision to “ground” (I think thats what it is called). Look in your scene outlier for “landscape” and check the details. Never made a map but since you asked for “any” input 
No it only happened suddenly… twice. I’ve managed to remedy the problem by copying over a backup of the landscape from the umap files I backed up earlier. So whatever the problem is… it’s directly related to the landscape.