Hey frezer,
I followed your steps but was unable to duplicate issue you described. Let me list my steps, and if anything seems different than what you did maybe we can narrow down source of problem.
- Create new Landscape
- Change Landscape Manage tool to Edit Splines
- CTRL + LMB click to add multiple splines (I made a simple circle)
- In Details panel, while still in Edit Splines mode, made sure Enable Collision was checked (it was enabled by default)
- Added a simple Cube Mesh to Spline
- Opened Mesh in Mesh Editor
- Changed Collision Complexity to Use Complex Collision As Simple
- Clicked Play
Doing it this way, my Mesh had collision working just fine. Did I miss any steps you took? Is there anything special about Mesh you are using? Any additional information we may be missing?
Best,