from 5.6 Release Notes, under “Modeling Tools” section :
https://dev.epicgames.com/documentation/en-us/unreal-engine/unreal-engine-5-6-release-notes#modelingtools:~:text=Modified%20the%20default,to%20generate%20collision.
// * Modified the default CollisionMode for new meshes to SimpleAndComplex.
- The previous default CollisionMode of UseComplexAsSimple was chosen to get a quick out of the box collision from generated meshes. Although this default works for out of the box collision, it does not work for physics simulations. It can be difficult to deduce why that is the case when you have collision.
- To make this easier to discover and more consistent with the rest of the engine, the default for new meshes is now SimpleAndComplex. Users can use the collision tools under the Attribs category of Modeling Mode to generate collision. //
If you don’t know what’s the difference between “Simple” and “Complex” collision - there is a doc page :
it’s old (from ue4 days) but nothing really changed about how they work.
You can change it back if you want to, switch to modeling mode > settings :
