I was just messing around in my test world again and found that slabs will take water from any level and waterlog itself. No other waterloggable blocks will do this other than all slab variants.

The first part of this video shows when I just recognized the bug and I just so happened to be recording at the time: https://www.dropbox.com/s/8ri1tm7p8uyh4lm/Auto-Logged.mp4?dl=0