Adding a component to a blueprint makes it a child of the root component. In 5.1, building the level breaks the component list so that all components become siblings, which transforms all components to their absolute location rather than relative to their parent.
Re-compiling the blueprint restores the hierarchy, but you can’t build the level without breaking it again. I’ve tested this on several projects, including the ‘Old West’ Unreal project.
I cannot migrate my project to 5.1 until this is fixed.
Seems like a major bug. Why can’t I find references to this problem anywhere? I filed a bug report, but no answer yet.