Unfortunately, glass is not compatible with decals, and probably never will be.
Man, I hate glass.
This is sort of hack-y way to get around this issue. Instead of using a decal, we can use a canvas sphere placed inside of a potion bottle, at a size just right so that it looks like it is on the bottle.
Here is the template: (1000x1000)
https://i.imgur.com/faGYLYL.png
The image is 1000 x 1000, pretty large but done so that it fits on the sphere right.
The ratio of the image is 1:2, so stretch your images accordingly!
If the potion bottle is 1.0 scale, I found the best scale for the sphere is:
X = 0.4 Y = 0.4 Z = 0.36
Place it in the same coordinates as the bottle. (or alternatively, if you are using masked instead of transparent, place the sphere 2.5 units lower on the Z axis, and set the scale to 0.44/0.44/0.42)
Here’s what the template looks like in-game:
Unfortunately, if you use the regular transparent, the liquid and glass renders over the image.
Here’s what the alternative masked version looks like (at the cost of transparency quality):
