Two different issues going on here.
The OP is reporting actual raindrops (such as those visibly falling from the sky) clipping through the Stormglass Sloped Roof (glass). None of the other sloped roof pieces allow the falling raindrops to pass through. This has been in game since Siptah was launched into early access and the Stormglass build set was made available.
The other issue, that I believe you and several others are commenting on, is a separate issue where all surfaces acquire the ‘wet’ appearance whenever it rains. This is, as far as I understand, a limitation with Unreal Engine in use. They can fix the indoor wet textures by turning off all wet textures. Everywhere. This my understanding of that issue. I could be mistaken.
Post from Sept 2020: Stormglass sloped ceiling does not stop rain