16x anti-aliasing not working

This has been affecting me for a while, but I only just got around to reporting it.

When I have anti-aliasing set to 16x, it simply doesn't do anything, exactly as if it was off. It works just fine at other levels. It works fine at 16x in other OpenGL applications.

I'm using Windows 10, an RX Vega 56 and AMD's proprietary Windows drivers.