When enabling improved glass a mild dark outline appears on distant glass. I tested the default settings with only improved glass enabled and experienced the same outcome. How noticeable it is depends slightly on the resource pack you're using and is less noticeable with the connected textures pack I was using before until I switched to vanilla to confirm it wasn't a resource pack issue.
Mesa 25.0.7-2
RX 9060XT
Minecraft 1.21.8
Iris 1.9.6
Complementary r5.6.1