I run a Minecraft server with NitrousNetworks and have disabled all plugins and shader packs (including Optifine) and updated to 1.12.2 in Spigot. When placing down half slabs, the item renders as a full block each time. This issue also affected 1.12.1 and even 1.12.
In a single player game (not on a server) in 1.12.2 with no shaders or mods installed, this issue also persists.
This issue started as soon as I began playing about 3 hours ago. This issue was not seen yesterday when I was playing about with half slabs so it is a very recent issue for me.
Just found out that placing a single slab onto grass will create two single slabs side by side.
In this screenshot provided below, placing down a half slab either places two slabs together or a full block. It seems to be random.
This issue has been resolved with a restart of my computer. As simple as that!