I created a 2-level landscape in World Partition, low on -640, high on +640. If I want to use Flatten (landscape mode) and set Flatten Target to the desired level, lowering the landscape to lowest level (-640) that’s OK, but raising low level to +640 won’t get any further than 0 - so only halfway, even when the grid is shown on +640.
Even when I locate the landscape to -640 - so the top is on 0) Flatten works only halfway up the height, so I think it’s not related to Z-axis of location, but to the height difference in the landscape itself