Models that use a transparent texture like stained glass are being incorrectly rendered in the latest version. There is no longer any transparency through the glass texture and it will just render fully opaque.
First image is from the same pack on 1.21.10, second is 1.21.11 RC-2
Can confirm this issue. It can be recreated without extra resource pack by using this command:
On 1.21.10 it will look the same as Red Stained Glass item, while on 1.21.11-rc2 it will look solid.
See image below (left is regular red stained glass, right is a stick with it’s model)