Hi Mechlul,
While I was not able to reproduce it on my end in a clean project, it clearly reproduces in the project sent. Unfortunately, I’m not certain what is occurring, but I have entered a bug report, UE-30032, to be assessed by the development staff. In the mean time, it would be best to use the Resampling feature frequently to ensure collision is maintained on your landscape while you work as a workaround until a fix can be implemented.