Water sometimes flows in unexpected directions

1. Place a water source block in a corner with flat ground extending from it.

What I expected to happen:
I expected that the water would flow out in a triangle-like pattern.

What actually happened:
The water initially started flowing out in a triangle-like pattern but skipped 3 blocks in the middle of the outer edge.

Workaround:
Placing a block in one of two specific locations in the flowing water pattern fixes the pattern, but must be done every single time a water source block is placed and allowed to flow out.