The bug
As can be seen through the screenshots, the grindstone has more than enough space to support these blocks. However, they cannot be placed on them without using /setblock (which I used for the images). Other blocks such as sea pickles, lanterns and torches can correctly be placed on top.
May be considered WAI, but felt this was worth reporting anyway given the changes to transparent blocks in recent snapshots.
Linked issues
relates to 9
MC-146824
Inconsistency: Ladders and tripwire hooks cannot be placed on the sides of redstone blocks, observers and target blocks
Resolved
MC-147252
Bells cannot be placed on top of cauldrons, composters, hoppers or other bells
Resolved
MC-131115
Torches, levers, buttons, tripwire hooks, coral fans, vines, ladders and bells cannot be placed on the sides of farmland or grass paths
Resolved
MC-131122
Torches can't be placed on iron bars or glass panes
Resolved
MC-146928
Can't place doors, rails, buttons, pressure plate, redstone, etc. on soul sand
Resolved
Attachments
Comments 20
This might not be a bug. It's a partial block and a machine. It doesn't have the ability to relay redstone signals through the block, and it doesn't by itself interact with redstone.
Grindstones are non-solid blocks that are not redstone components. This is intended, so far as I know.
I think this works as intended, because it seems things like torches are an exception, where they are allowed to be placed as long as there's enough room. Everything else has to have a full face. However, I agree that this should be changed so that all those blocks can be placed as long as there's enough room, but that is a feature request, not a bug.
It kinda make sense, because the grindstone turns around.. (Mby it should be, that you can place it on top, but once someone use it, they pop-off)