On the Vibrant Visuals preview, stained glass only has an outline and is otherwise fully transparent. This is probably unintentional behaviour since other blocks like nether portals are still correctly translucent.
Linked issues
is duplicated by 4
MCPE-199261
Vibrant Visuals: Stained glass is too transparent.
Resolved
MCPE-201786
Vibrant visual - colored glass doesn't have colors
Resolved
MCPE-201815
Glass Is Too Transparent in Vibrant Visuals, Making Buildings Look Unfinished
Resolved
MCPE-202030
Stained Glass Insides not rendering properly (Vibrant Visuals)
Resolved
Also affects tinted glass.
[media]