Basically, I was making a redstone system where dispensers place and remove water in stairs.
The dispensers were perfectly fine when they placed water in the stairs, making them waterlogged, and the water flooded in as expected. However, when the dispensers sucked up the water, it sucked up the stair, too.

Basically, the code makes the dispenser delete the block in front of it, rather than turning waterlogged to false. Hopefully the programmers keep this in mind for anything with the waterlogged tag.

Note: Water stops flowing over non-waterlogged stairs that are upside-up. [Upside-Down works fine] I used this to prevent it from flooding the wrong way.