The texture of the 'Potion Of Weakness' is very hard to distinguish from an Glass Bottle. This is only the case in your hotbar when using the default Minecraft textures.
MC-260703
Potion of Weakness texture color is hard to distinguish from regular glass bottles when in the hotbar
Resolved
Won't Fix
0
Confirmed, though this may be intended. The color of the potion of weakness is indeed very similar to the hotbar background color, therefore making it hard to tell whether the bottle is a potion of weakness or empty.