Bump. Thanks for reporting. Just wanted to confirm, that this UI bug still exists.
Quite confusing when starting with a 2D guide layer and clicking the 3D icon and nothing happens. When having the layer controls on, the user can see the 3D gizmo appear, but having the layer controls hidden, the user has to expand the transform property to check if the guide layer is 3D or not.
System info Application: After Effects (Beta) v184.108.40.206 OS: Windows v10.0.19042, RAM: 63.95 GB, CPUs (logical): 24