As I was reducing the project down to post for troubleshooting, I noticed that not only was a CameraObject being spawned automatically, but so was a DefaultPawn.
I added a GetAllActors in a test scene to find all the CameraObjects, and called destroy on all but the camera attached to my character pawn. The sphere was still there. When I repeated this for all DefaultPawn objects, the sphere was gone. Clicking the created DefaultPawn object in the world outliner while running the game in PIE shows its origin as exactly where the sphere I observed would be centered.
It turns out the game mode was set to the default ‘GameMode’ rather than the custom one I created, so this isn’t so much a bug as confusion on my part.
As a quality of life suggestion: when a project specifies a game mode that can not be found (as in my case where the config files copied over to the new project specified a custom game mode, but the game mode definition was not considered a dependency of the maps I migrated and thus were not in the content directory) it should report the error in the editor window.
Thank you for responding,