I have now tested the affected alirium grass (e.g. Downy Grass) with three different viewers:
• Second Life Viewer 26.3.0.31203661088: grass renders correctly
• Firestorm 7.2.4.80712: grass renders correctly
• Second Life Viewer 26.4.0.34502396632: grass is completely invisible
I tested the same objects at the same location without making any changes to them.
This therefore appears to be a rendering regression introduced in the Second Life 26.4 viewer.
The affected alirium grass uses older vegetation textures with transparency/alpha.
I am currently staying on 26.3.0.31203661088, where all of these objects render correctly.