Does this happen to new shaders? Did your old shader use tessellation/Displacement?

Odd! Will look into it

No problem! Glad it worked out :)

How would you use vertex position for alpha in this case?

If you want the vertex position, you could simply use the World Position node, which, when used in the vertex shader, is the vertex position, otherwise it's the fragment position.

If you want to have transparency based on vertex painted colors, use the vertex color node and connect it to the Alpha.

Currently, screen coordinates are per-vertex. I'll add an option to select per-fragment instead

That's how screen coordinates work, 0 is in the center, so it's not a bug!

However, there could be a feature to make it 0 to 1 instead of -1 to 1, in that dropdown menu. Open a request for a feature like that if you want :)