Icons being cropped in-game

[GotLag]

I've found that custom icons for splitters are being cropped to 16 pixels high (centred on the 32 x 32 icon image). I can't find anything in the prototype definitions for the relevant entities, items or recipes that would cause this.

My in-game toolbar (the same problem occurs in the inventory and crafting screens):


The icon files I'm using:

What's going on?