The default scene root of an blueprint actor is not removed after adding a static mesh component via C++ until after restarting the project (or reloading the blueprint asset)
Found in 4.22 CL#7053642
Reproduced in 4.21 CL#4753647, 4.24Main CL#7206954
In the Details panel, the static mesh component is added to the component hierarchy and DefaultSceneRoot is nested under it.
The DefaultSceneRoot to be removed.
Reload the blueprint asset. This will remove the DefaultSceneRoot.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-76806 in the post.