Thanks for the thought! But I finally did notice the issue. BP_Flyer was used by PlayerStart, and the added collider on the sphere (or cone) was leading to an intersection with another collider on startup, so Unreal was removing the PlayerStart pawn.
I hadn’t noticed sooner because I don’t actually use colliders on the character (intentionally). Adding the sphere’s collider was just a mistake and disabling it was all I needed, but I still needed to understand why that had caused issues.