Isn't this normal vanilla behavior? I'm not doing anything special for block placement.

The only thing that might be unexpected is the facing direction of the panel, which is due to the fact that it's based on the side of the grass clicked. Do you think this a big enough problem to justify changing the facing calculations?

It's just very very unusual. But well, there is currently no vanilla minecraft block that can be placed on the side of a grass block. But I think it's okay like that.