Water fails to flow to next priority if primary route of attempting to create source in waterlogged block fails

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.