A plane is only a one-sided object to AC, so when AC renders it, it only shows the side that in Blender, when you select Face Orientation, would be blue. If you select Face Orientation currently in Blender, the side that's "invisible" appears as red. To remedy this, you can, in Edit Mode, create a duplicate of the mesh (Shift-D), then Flip Normals (I hit F3 and type Flip Normals). In Face Orientation mode, it would show as a darker red/fuschia colour at this stage. This allows AC to register both a front/back so it can be viewed from either side.