According to the texture assignment in the 'blocks.json' file, both the bottom and the top texture will be the 'east' texture, whereas the normal texture for the sides is 'down'. This is not only irritating but also limits the creators of texture and resource packs as they'll have to go all-in or nothing. Splitting bottom and top textures and assigning them in a more logical way would not only be easier to understand but also allow for more customization for content and resource pack creators
 
 
"stained_glass_pane": {
"blockshape": "double_side_fence",
"textures": {
"up": "stained_glass",
"down": "stained_glass",
"north": "stained_glass",
"south": "stained_glass",
"west": "stained_glass",
"east": "stained_glass_pane_top"
},
"sound": "glass"
},