CTRL+G is currently acting weird, you may need to use the context menu to group objects

Grab a few, group them, then click "Undo". The objects may disappear. Their render state seems to be broken, moving the camera around they will show up somethings, as if their culling position is incorrect and they are culling when you are close to them.

May depend on where the object is in the scene. Probably only happens for static objects (not sure).