Not sure if this is somehow related, but I found collision breaks after adding or removing landscape components. The fix is to change the Cell Size in the World Settings (under the World Partition heading). Instead of 12800, try power of 2 values, such as 8192 or 16384 (although other non-power of 2 values appear to still work). This also can solve the issue with landscape streaming proxies not loading when the player is close to their boundary (however, this fix has not been tested in an actual build, just in-Editor).