You are using the RTX Realtime mode - there is a cutoff threshold for opacity in this mode, that is set to 1. If you change to path-traced you’ll have the full range of opacity available. To use Opacity in Realtime mode, go to the render settings tab on the right panel, and on Translucency check the “Enable Fractional cutout Opacity”. Results will be subpar, though:
Ideally, you should use a translucent material. For best results you should use RTX - Interactive (Path traced)
Two images for comparison using a Omniglass, and changing the Glass Roughgness to 0.28
