I'm not sure how to explain this one haha. You place a snow layer down where there is short grass, the short grass turns into a fence post. It even has fence post collision as well. However I cannot break the fence post with an axe, it does not let me mine or touch it. Breaking the snow layer will turn the fence post back into short grass.

I run optifine and shaders and I'm wondering if it's some weird compatibility issue there, I'll try to do some more testing to see if I can narrow down what is causing this.

EDIT: I forgot to mention, this is for the latest version on 1.12.2

just making a note, not an incompatibility with optifine, same thing happens without it.