The Bug

Water attempts to flow through waterlogged blocks, but can't (as it won't create a source), but doesn't try the "longer" route if the route is over 2 blocks.

How to reproduce

1. Create a platform like the one shown in 2018-05-31_12.23.28.png (below)
2. Place water next to the 2 slabs

The slabs can be replaced with any waterlogged block: stairs, chests, ladders.