As I pointed out there are known issues with landscapes collision detection but no news on when those will be update or fixed. As a work around you could always use blocking volumes which handle collision better, but beyond that I’m not sure what to tell you. 
If you’re using any Engine Source and want to implement a plugin I read earlier that Rama has released a plugin on the forums that is really a “work around plugin” to enhance how collision is handled specifically for landscapes. I cannot tell you how good it works but he does have videos demonstrating it’s capabilities. Forum Post Here
Tim