Hello ,
I created a new clean, blank project and was unable to reproduce this issue no matter what, which seems to point the finger to my main project.
On my project where this occurs it seems like all I have to do to break the collision again on that specific spot, is to paint a new texture over it.
The landscape was made in the editor and was created on version 4.9 and imported to 4.11.
Would you like me to send you the part of the landscape that is broken or is there some sort of debug tool where I could provide some additional information?
Kind regards,