Current branch Creative mode, observed in RE, expecting same in vanilla minor issue (though with the new explosions...? didnt test if it passed through) they seem to be missing one pixel, leaving a tiny gap between them and the block next to them. put two next to each other to make easier to see
While you're looking at that block, can you also check into why certain faces of the half-block portion become unpaintable when the block is in certain orientations? The texture tool seems to just miss half the time.