Go to Settings / Miscellaneous and turn off Show Warning: Group Header removed from Content. By default, the shy'd layer above the group header is turned on, so it can display the said error message. However, since the layer is enabled (but typically set to 0% opacity), it will break AE's 3D space. By deactivating this setting (and thereby the layer), we can retain the 3D space. Just be aware that you won't get an error message anymore when moving group layers illegally.