Quick answer: Unreal Static Mesh used only via ISM cooked without collision? Engine optimizes 'never directly referenced' meshes; ISM use isn't a direct ref - mark CollisionTraceFlag explicitly.
ISM-spawned trees have no collision in shipping build. Editor PIE works.
Force collision
StaticMesh->CreatePhysicsMeshes();Generate collision data in editor; ship-included.
Or hard-reference once
Place one tree as a regular static mesh actor in a level. Cooker treats it as referenced; collision survives.
Set CollisionTraceFlag UseComplexAsSimple
Marks mesh as collision-significant. Cooker preserves the data.
“Cooker tree-shakes collision aggressively. Marks of usage need to be explicit.”
For ISM-heavy worlds, the collision-cooking bug is a class. Validate post-cook; visible only on device.