I had a similar problem in 4.18. I say similar rather than exactly the same as I didn’t try exporting my landscape like you did so I can’t be 100% sure I had the size issue, although my error message was the same. What fixed the error message for me was to import the lanscape via Tiled Landscape rather than creating each individual level and adding the landscape via heigtmap. When I originally did the latter I found that I could only spawn foliage on my landscape level that was positioned at 0,0 coordinates. If I moved the position of the landscape or tried painting foliage on the other levels it would give the same error message that you have. Once I imported the levels via tiled landscape it all just worked. Hope this helps.